Add a linter for Apache Thrift IDL files

This linter works by invoking the `thrift` compiler with the buffer
contents and reporting any parser and code generation issues.

The handler rolls its own output-matching loop because we have the
(unfortunate) requirement of handling error output that spans multiple
lines.

Unit tests cover both the command callback and handler, and there is
initial documentation for all of the option variables.

test/handler/test_thrift_handler.vader

@@ -0,0 +1,63 @@
+Before:
+    runtime ale_linters/thrift/thrift.vim
+
+After:
+    call ale#linter#Reset()
+
+Execute(The thrift handler should handle basic warnings and errors):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 17,
+  \     'col': 0,
+  \     'type': 'W',
+  \     'text': 'The "byte" type is a compatibility alias for "i8". Use i8" to emphasize the signedness of this type.',
+  \   },
+  \   {
+  \     'lnum': 20,
+  \     'col': 0,
+  \     'type': 'W',
+  \     'text': 'Could not find include file include.thrift',
+  \   },
+  \   {
+  \     'lnum': 83,
+  \     'col': 0,
+  \     'type': 'E',
+  \     'text': 'Enum FOO is already defined!',
+  \   },
+  \ ],
+  \ ale_linters#thrift#thrift#Handle(1, [
+  \   '[WARNING:/path/filename.thrift:17] The "byte" type is a compatibility alias for "i8". Use i8" to emphasize the signedness of this type.',
+  \   '[WARNING:/path/filename.thrift:20] Could not find include file include.thrift',
+  \   '[FAILURE:/path/filename.thrift:83] Enum FOO is already defined!',
+  \ ])
+
+Execute(The thrift handler should handle multiline errors):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 75,
+  \     'col': 0,
+  \     'type': 'E',
+  \     'text': 'This integer is too big: "11111111114213213453243"',
+  \   },
+  \   {
+  \     'lnum': 76,
+  \     'col': 0,
+  \     'type': 'E',
+  \     'text': 'Implicit field keys are deprecated and not allowed with -strict',
+  \   },
+  \   {
+  \     'lnum': 77,
+  \     'col': 0,
+  \     'type': 'E',
+  \     'text': "Unknown error (last token was ';')",
+  \   },
+  \ ],
+  \ ale_linters#thrift#thrift#Handle(1, [
+  \   "[ERROR:/path/filename.thrift:75] (last token was '11111111114213213453243')",
+  \   'This integer is too big: "11111111114213213453243"',
+  \   "[ERROR:/path/filename.thrift:76] (last token was ';')",
+  \   'Implicit field keys are deprecated and not allowed with -strict',
+  \   "[ERROR:/path/filename.thrift:77] (last token was ';')",
+  \ ])