The current help output of kakoune doesn't mention that a double dash
(`--`) can be used to separate the files from options.

This special case was not handled in the `ParametersParser` and
therefore did not respect the `--` separator for switches.

Adding the `WithCoord` flag will make the `ParametersParser` try to
parse a special `+<line>:<column>` VIM-style switch.

This moves the parameter parsing for the `+<line>:<column>` switch
of the commandline to the `ParametersParser`. The position switch now
respects the `--` separator between switches and files.

The manpage does not document the old behaviour of `+:[column]`.
`+:[column]` works exactly like `+`. This is compatible with the
previously documented behaviour for `+:`.

The help message is does not mention that omitting the line or column
moves the cursor to the last line or column. The explanation in the
manpage should suffice.