add option --mixin-files

[dirk-thomas]

The new option enables to pass custom .mixin files without the need to place them under the $COLCON_HOME/mixin directory. The passed mixin files can also overlay any existing mixin to e.g. test modifications before deploying them through a mixin repository.

[dirk-thomas]

This change breaks --mixin.

When parse_known_args is being invoked to extract additional mixin files only --mixin-files is present but not --args yet. So when passing --mixin the argument is being considered to match --mixin-files (based on the unique prefix) and the argument likely fails the type check.