Add fsc linter for Scala (#1452)

* Add fsc as a Scala linter

* Pull reused code into `autoload/ale/` directory

* Include fsc into the README

* Add unit test for testing the scala handler

* Add unit test for scala's fsc linter

* Rename scala unit tests for clarity

* Fix typo in README

* Fix typos in doc/ale.txt

* Fix author headline

* Put methods for fsc commands back into fsc.vim

* Move command_callback tests to correct location

* Rewrite handler test so it actually tests handler

* Clarify description of test in test_scala_handler

doc/ale.txt

@@ -378,10 +378,10 @@ Notes:
 * Rust: `cargo`!!, `rls`, `rustc` (see |ale-integration-rust|), `rustfmt`
 * SASS: `sass-lint`, `stylelint`
 * SCSS: `prettier`, `sass-lint`, `scss-lint`, `stylelint`
-* Scala: `scalac`, `scalastyle`
+* Scala: `fsc`, `scalac`, `scalastyle`
 * Slim: `slim-lint`
 * SML: `smlnj`
-* Solidity: `solhint, solium`
+* Solidity: `solhint`, `solium`
 * Stylus: `stylelint`
 * SQL: `sqlint`
 * Swift: `swiftlint`, `swiftformat`