add crystal lint

- invokes via `crystal build` command without codegen
- adds vader tests

ale_linters/crystal/crystal.vim

@@ -0,0 +1,40 @@
+" Author: Jordan Andree <https://github.com/jordanandree>
+" Description: This file adds support for checking Crystal with crystal build
+
+function! ale_linters#crystal#crystal#Handle(buffer, lines) abort
+    let l:output = []
+
+    let l:lines = join(a:lines, '')
+    
+    if !empty(l:lines)
+      let l:errors = json_decode(l:lines)
+
+      for l:error in l:errors
+          call add(l:output, {
+          \   'bufnr': a:buffer,
+          \   'lnum': l:error.line + 0,
+          \   'col': l:error.column + 0,
+          \   'text': l:error.message,
+          \   'type': 'E',
+          \})
+      endfor
+    endif
+
+    return l:output
+endfunction
+
+function! ale_linters#crystal#crystal#GetCommand(buffer) abort
+  let l:crystal_cmd = 'crystal build -f json --no-codegen -o '
+  let l:crystal_cmd .= shellescape(g:ale#util#nul_file)
+  let l:crystal_cmd .= ' %t'
+
+  return l:crystal_cmd
+endfunction
+
+call ale#linter#Define('crystal', {
+\   'name': 'crystal',
+\   'executable': 'crystal',
+\   'output_stream': 'both',
+\   'command_callback': 'ale_linters#crystal#crystal#GetCommand',
+\   'callback': 'ale_linters#crystal#crystal#Handle',
+\})