Added support for user define custom actions

docs/custom-actions.md

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+# Cursorless custom VSCode actions
+
+You can use Cursorless to run any built in VSCode command on a specific target.
+
+Just add your custom commands to: `actions_custom.csv`
+
+```csv
+Spoken form, VSCode command
+custom, editor.action.addCommentLine
+```
+
+You can now use the command: `custom blue air` to comment the line containing the token with the `blue a` hat. 

docs/public-api.md

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+# Cursorless public API
+
+## Public Talon captures
+* `<user.cursorless_target>`    
+    Represents a cursorless target. This target can be a single mark, multiple marks or a range.
+
+## Public Talon actions
+* `user.cursorless_command(action_id: str, target: cursorless_target)`    
+    Perform a Cursorless command on the given target    
+    eg: `user.cursorless_command("setSelection", cursorless_target)`
+* `user.cursorless_vscode_command(command_id: str, target: cursorless_target)`    
+    Performs a VSCode command on the given target    
+    eg: `user.cursorless_vscode_command("editor.action.addCommentLine", cursorless_target)`
+
+## Example of combining capture and action
+```talon
+select <user.cursorless_target>:
+    user.cursorless_command("setSelection", cursorless_target)
+
+comment <user.cursorless_target>:
+    user.cursorless_vscode_command("editor.action.addCommentLine", cursorless_target)
+```