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

ale_linters/scala/fsc.vim

@@ -0,0 +1,29 @@
+" Author: Nils Leuzinger - https://github.com/PawkyPenguin
+" Description: Basic scala support using fsc
+"
+function! ale_linters#scala#fsc#GetExecutable(buffer) abort
+    if index(split(getbufvar(a:buffer, '&filetype'), '\.'), 'sbt') >= 0
+        " Don't check sbt files
+        return ''
+    endif
+
+    return 'fsc'
+endfunction
+
+function! ale_linters#scala#fsc#GetCommand(buffer) abort
+    let l:executable = ale_linters#scala#fsc#GetExecutable(a:buffer)
+
+    if empty(l:executable)
+        return ''
+    endif
+
+    return ale#Escape(l:executable) . ' -Ystop-after:parser %t'
+endfunction
+
+call ale#linter#Define('scala', {
+\   'name': 'fsc',
+\   'executable_callback': 'ale_linters#scala#fsc#GetExecutable',
+\   'command_callback': 'ale_linters#scala#fsc#GetCommand',
+\   'callback': 'ale#handlers#scala#HandleScalacLintFormat',
+\   'output_stream': 'stderr',
+\})