Add support for NumericUnderscores extensions from CLI/config

[9999years]

Closes #1434

Supercedes #1435, updated for latest master.

[zliu41]

I'll defer this to @ndmitchell. I think the intention of passing extensions to HLint is for parsing, not for deciding if a hint should trigger, although it seems reasonable to do the latter.

Even if we decide to do that, this PR will need a few more updates:

• I'm not seeing how you are passing the additional extensions from the Yaml config to the places that need them.
• Since the additional extensions are from the HLint config and not tied to individual modules, it doesn't seem to make sense to associate them with ModuleEx.
• Need to add some tests.