Allow external fixers on command-line tool

[shaib]

The section on Fixers at https://python-modernize.readthedocs.io/en/latest/fixers.html ends with a note about "implementing your own fixers"; however, libmodernize.main.main collects the available fixers from libmodernize.fixes and lib2to3.fixes, and if a fixer not found in these two places is specified on the command line, errors out with "Error: fix '{}' was not found".

It would be more helpful to try to import and load the fixer in such cases. if a fix explicitly mentioned was not found, an attempt should be made to import and load it.

I note that modernize has inherited this behavior from 2to3 -- but the only reason I see for it is to make sure that -l indeed provides a full list; a problem which could be addressed by changing that option's help-string, IMO.

[takluyver]

Makes sense, PRs welcome :-)

[shaib]

Just for the record, #167