Add a "Select formatter" command

[brettcannon]

Ala Select linter, list the formatters we support and allow a user to select one that we then activate as appropriate. Provides an easy way for people to know what formatters we support and to get them on to their machine.

[MikhailArkhipov]

We may actually remove them eventually. VS engine provides formatting. May not be in the first release, but it does exist.

#1011

[DonJayamanne]

I don't think we should remove this at all. We need to keep supporting external tools for formatting. People have formatting rules in Python that use these existing tools.
The ptvs formatter could be another option.
For instance pep8 follows the pep8 standards for formatting. I'd consider that to be the best practice.

[brettcannon]

Right, while we would provide formatting via the analysis engine out of the box we will continue to support third-party formatters for those people who have their internal style guides set up for those tools.

[luabud]

Related #5121

[DonJayamanne]

Closing in favor of #5121