Add executable option for phpmd linter (resolves #1076) (#1078)

* Add executable option for phpmd linter (resolves #1076)
* Add test for phpmd executable option

ale_linters/php/phpmd.vim

@@ -1,11 +1,20 @@
-" Author: medains <https://github.com/medains>
+" Author: medains <https://github.com/medains>, David Sierra <https://github.com/davidsierradz>
 " Description: phpmd for PHP files
 
+let g:ale_php_phpmd_executable = get(g:, 'ale_php_phpmd_executable', 'phpmd')
+
 " Set to change the ruleset
 let g:ale_php_phpmd_ruleset = get(g:, 'ale_php_phpmd_ruleset', 'cleancode,codesize,controversial,design,naming,unusedcode')
 
+function! ale_linters#php#phpmd#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'php_phpmd_executable')
+endfunction
+
 function! ale_linters#php#phpmd#GetCommand(buffer) abort
-    return 'phpmd %s text '
+    let l:executable = ale_linters#php#phpmd#GetExecutable(a:buffer)
+
+    return ale#Escape(l:executable)
+    \   . ' %s text '
     \   . ale#Var(a:buffer, 'php_phpmd_ruleset')
     \   . ' --ignore-violations-on-exit %t'
 endfunction
@@ -30,7 +39,7 @@ endfunction
 
 call ale#linter#Define('php', {
 \   'name': 'phpmd',
-\   'executable': 'phpmd',
+\   'executable_callback': 'ale_linters#php#phpmd#GetExecutable',
 \   'command_callback': 'ale_linters#php#phpmd#GetCommand',
 \   'callback': 'ale_linters#php#phpmd#Handle',
 \})