Run common checks for every single solution; Add common check for snake_case

[angelikatyborska]

This is my attempt at adding common checks that will be done on every single solution, as suggested in #97

For starters, I'm adding two checks that were pretty easy to implement:

• module attribute names should be snake_case
• function names should be snake_case

I can also see that it should be relatively easy to implement a check for PascalCase module names. It might be a bit more difficult to check for snake_case variable names, but still worth it.

I would also want to later add a check for not using tabs for indentation, but only as a informative comment not to be too annoying.

Some checks can be "stolen" from Credo :)

• module names: https://github.com/rrrene/credo/blob/master/lib/credo/check/readability/module_names.ex
• snake case variable names: https://github.com/rrrene/credo/blob/master/lib/credo/check/readability/variable_names.ex
• predicate function names (for Bob!): https://github.com/rrrene/credo/blob/master/lib/credo/check/readability/predicate_function_names.ex
• @impl ModuleName instead of @impl true: https://github.com/rrrene/credo/blob/master/lib/credo/check/readability/impl_true.ex

Website copy PR: exercism/website-copy#2019

[angelikatyborska]

@jiegillet your review would also be much appreciated!

[neenjaw]

100% agree, despite the warning on the function not to use, I see it used everywhere in non-core code :P

[neenjaw]

Same simplification could be done here

[angelikatyborska]

I'll merge to see it in action and add more checks in new PR(s)