Allow user to specify source command

[Flet]

Hello!

There is currently support for semistandard via the standad.semistandard setting, which is great!

However, it would be really nice to add a setting to specify a custom source command. This would allow the plugin to work with any standard-engine flavored command.

The main goal here is to allow folks to use https://github.com/standard/standardx with this plugin.

Doing it via a custom command enables even more flexibility for other commands (doublestandard is always a good choice too).

[LinusU]

Would love to see automatic detection as well. Currently, when modifying any of e.g. Sindre's modules, basically every line shows up as an error. It would be cool if this module could detect xo in devDependencies, and then automatically use xo for that project.

[johnelliott]

One of our devs would like to do this too 👍

[xverges]

Would love to see #71 merged!