Support optin config with globs

[Raynos]

Currently there are three modes to run the checker:

• jsig . which runs the checker against all code.
• jsig file.js which runs a single file
• jsig . --optin which runs the checker against all code, but only files with /* @jsig */

I'd be good to an option to pass:

• jsig . --files glob --files glob --files glob
• or in the jsigconfig, { files: [ glob, glob, glob ] }

This allows for a package to configure the files to be checked like they would configure a linter with the files to lint.