Add a script for custom checks to enforce using the abort flag for functions and trailing whitespace, and fix existing issues.

ale_linters/javascript/eslint.vim

@@ -28,7 +28,7 @@ function! ale_linters#javascript#eslint#GetCommand(buffer) abort
     \   . ' -f unix --stdin --stdin-filename %s'
 endfunction
 
-function! ale_linters#javascript#eslint#Handle(buffer, lines)
+function! ale_linters#javascript#eslint#Handle(buffer, lines) abort
     " Matches patterns line the following:
     "
     " /path/to/some-filename.js:47:14: Missing trailing comma. [Warning/comma-dangle]

ale_linters/javascript/flow.vim

@@ -24,7 +24,7 @@ function! ale_linters#javascript#flow#GetCommand(buffer) abort
   \   . ' check-contents --respect-pragma --json --from ale %s'
 endfunction
 
-function! ale_linters#javascript#flow#Handle(buffer, lines)
+function! ale_linters#javascript#flow#Handle(buffer, lines) abort
   let l:str = join(a:lines, '')
   if l:str ==# ''
     return []

ale_linters/javascript/jshint.vim

@@ -19,7 +19,7 @@ function! ale_linters#javascript#jshint#GetExecutable(buffer) abort
     \)
 endfunction
 
-function! ale_linters#javascript#jshint#GetCommand(buffer)
+function! ale_linters#javascript#jshint#GetCommand(buffer) abort
     " Search for a local JShint config locaation, and default to a global one.
     let l:jshint_config = ale#util#ResolveLocalPath(
     \   a:buffer,