Allow for alternate restyled config locations

[chiroptical]

This PR allows for alternate restyled.yaml configuration files closing #98

I formatted the code with brittany using your configuration file.

I tried to write the most general findJust function possible. I tried two versions shown here: https://gist.github.com/chiroptical/2a921faa08976f6a176557fe57bb6021.

I would like to add a test to make sure it is working correctly.

[CLAassistant]

All committers have signed the CLA.

[pbrisbin]

Thanks for this!

[pbrisbin]

For testing, I think you should start here https://github.com/restyled-io/restyler/blob/master/test/Restyler/ConfigSpec.hs#L20.

Apologies for not test-compiling anything I'm about to write, but I'd imagine you could do something like:

spec = do
  it "whatever" $ do
    app <- liftIO $ testApp "/" [("/a", "Some YAML"), ("/b", "Other YAML")]
    x <- runRIO app
        $ tryTo showConfigError
        $ loadConfigFrom ["/c", "/b"]
        $ const
        $ pure testRestylers

    -- assert you loaded /b, since /c didn't exit

    y <- runRIO app $ do
        writeFileUnreadable "/x" -- if /x is read, you will explode
        tryTo showConfigError
          $ loadConfigFrom ["/a", "/x"]
          $ const
          $ pure testRestylers

   -- assert you loaded /a and did not look at /x

[pbrisbin]

This LGTM!

Did you want me to wait for tests before merging, or add them myself after?

[chiroptical]

It took me a bit to figure out how I actually make this work. Let me know what you think of these. One other trivial test (if you think it is necessary) would be to add two configs to the test filesystem and make sure it reads the "first" one.

[pbrisbin]

Thanks, this looks great! I'm going to pull it into another branch and do some minor tweaks before merging.

[chiroptical]

It looks like it wasn't too much extra work to clean this up, thanks! See #103. I guess this can be closed.