rc: allow setting rc config and filter options as flat parameters

This commit allows global config options to be set as flat parameters on all rc
commands. This makes the rc commands much more like command line parameters and
will aid understanding of how the rc is used.

It is backwards compatible with the old method using _config and _filter.

docs/content/rc.md

@@ -404,6 +404,8 @@ This shows:
 If you wish to set config (the equivalent of the global flags) for the
 duration of an rc call only then pass in the `_config` parameter.
 
+Alternatively, you can pass config options flat at the top level of the parameter map. The option names are the same as their CLI flags without `--` and with `-` replaced by `_` (e.g. `transfers` instead of `Transfers` inside `_config`).
+
 This should be in the same format as the `main` key returned by
 [options/get](#options-get).
 
@@ -425,12 +427,26 @@ parameter, you would pass this parameter in your JSON blob.
 "_config":{"CheckSum": true}
 ```
 
+Or pass it flat at the top level:
+
+```json
+"checksum": true
+```
+
 If using `rclone rc` this could be passed as
 
 ```console
 rclone rc sync/sync ... _config='{"CheckSum": true}'
 ```
 
+Or simply flat:
+
+```console
+rclone rc sync/sync ... checksum=true
+```
+
+If both flat parameters and `_config` are supplied, the parameters in the legacy `_config` block will take precedence.
+
 Any config parameters you don't set will inherit the global defaults
 which were set with command line flags or environment variables.
 
@@ -443,6 +459,13 @@ setting the equivalent of `--buffer-size` in string or integer format.
 "_config":{"BufferSize": 44040192}
 ```
 
+Or flat:
+
+```json
+"buffer_size": "42M"
+"buffer_size": 44040192
+```
+
 If you wish to check the `_config` assignment has worked properly then
 calling `options/local` will show what the value got set to.
 
@@ -451,6 +474,8 @@ calling `options/local` will show what the value got set to.
 If you wish to set filters for the duration of an rc call only then
 pass in the `_filter` parameter.
 
+Alternatively, you can pass filter options flat at the top level of the parameter map. The option names are the same as their CLI flags without `--` and with `-` replaced by `_` (e.g. `exclude` instead of `Exclude` inside `_filter`).
+
 This should be in the same format as the `filter` key returned by
 [options/get](#options-get).
 
@@ -477,12 +502,26 @@ you would pass this parameter in your JSON blob.
 "_filter":{"MaxSize":"1M", "IncludeRule":["a","b"], "MaxAge":"42s"}
 ```
 
+Or pass them flat at the top level:
+
+```json
+"max_size":"1M", "include":["a","b"], "max_age":"42s"
+```
+
 If using `rclone rc` this could be passed as
 
 ```console
 rclone rc ... _filter='{"MaxSize":"1M", "IncludeRule":["a","b"], "MaxAge":"42s"}'
 ```
 
+Or simply flat:
+
+```console
+rclone rc ... max_size=1M include="a,b" max_age=42s
+```
+
+If both flat parameters and `_filter` are supplied, the parameters in the legacy `_filter` block will take precedence.
+
 Any filter parameters you don't set will inherit the global defaults
 which were set with command line flags or environment variables.
 
@@ -495,6 +534,12 @@ setting the equivalent of `--buffer-size` in string or integer format.
 "_filter":{"MinSize": 44040192}
 ```
 
+Or flat:
+
+```json
+"min_size": "42M"
+```
+
 If you wish to check the `_filter` assignment has worked properly then
 calling `options/local` will show what the value got set to.