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/command_callback/test_thrift_command_callback.vader

@@ -0,0 +1,61 @@
+Before:
+  Save g:ale_thrift_thrift_executable
+  Save g:ale_thrift_thrift_generators
+  Save g:ale_thrift_thrift_includes
+  Save g:ale_thrift_thrift_options
+
+  unlet! b:ale_thrift_thrift_executable
+  unlet! b:ale_thrift_thrift_generators
+  unlet! b:ale_thrift_thrift_includes
+  unlet! b:ale_thrift_thrift_options
+
+  function! GetCommand(buffer) abort
+    call ale#engine#InitBufferInfo(a:buffer)
+    let l:result = ale_linters#thrift#thrift#GetCommand(a:buffer)
+    call ale#engine#Cleanup(a:buffer)
+    return l:result
+  endfunction
+
+  runtime ale_linters/thrift/thrift.vim
+
+After:
+  Restore
+  delfunction GetCommand
+  unlet! b:ale_thrift_thrift_executable
+  unlet! b:ale_thrift_thrift_generators
+  unlet! b:ale_thrift_thrift_includes
+  unlet! b:ale_thrift_thrift_options
+  call ale#linter#Reset()
+
+Execute(The executable should be configurable):
+  AssertEqual 'thrift', ale_linters#thrift#thrift#GetExecutable(bufnr(''))
+
+  let b:ale_thrift_thrift_executable = 'foobar'
+  AssertEqual 'foobar', ale_linters#thrift#thrift#GetExecutable(bufnr(''))
+
+Execute(The executable should be used in the command):
+  Assert GetCommand(bufnr('%')) =~# "^'thrift'"
+
+  let b:ale_thrift_thrift_executable = 'foobar'
+  Assert GetCommand(bufnr('%')) =~# "^'foobar'"
+
+Execute(The list of generators should be configurable):
+  Assert GetCommand(bufnr('%')) =~# '--gen cpp'
+
+  let b:ale_thrift_thrift_generators = ['java', 'py:dynamic']
+  Assert GetCommand(bufnr('%')) =~# '--gen java --gen py:dynamic'
+
+  let b:ale_thrift_thrift_generators = []
+  Assert GetCommand(bufnr('%')) =~# '--gen cpp'
+
+Execute(The list of include paths should be configurable):
+  Assert GetCommand(bufnr('%')) !~# '-I'
+
+  let b:ale_thrift_thrift_includes = ['included/path']
+  Assert GetCommand(bufnr('%')) =~# '-I included/path'
+
+Execute(The string of compiler options should be configurable):
+  Assert GetCommand(bufnr('%')) =~# '-strict'
+
+  let b:ale_thrift_thrift_options = '-strict --allow-64bit-consts'
+  Assert GetCommand(bufnr('%')) =~# '-strict --allow-64bit-consts'