Use some default regex for completion support in other filetypes

autoload/ale/completion.vim

@@ -12,14 +12,14 @@ function! s:GetRegex(map, filetype) abort
         endif
     endfor
 
-    return ''
+    " Use the default regex for other files.
+    return s:should_complete_map['<default>']
 endfunction
 
 " Regular expressions for checking the characters in the line before where
 " the insert cursor is. If one of these matches, we'll check for completions.
 let s:should_complete_map = {
-\   'javascript': '\v[a-zA-Z$_][a-zA-Z$_0-9]*$|\.$',
-\   'typescript': '\v[a-zA-Z$_][a-zA-Z$_0-9]*$|\.$',
+\   '<default>': '\v[a-zA-Z$_][a-zA-Z$_0-9]*$|\.$',
 \}
 
 " Check if we should look for completions for a language.

test/test_completion.vader

@@ -230,6 +230,9 @@ Execute(Completion should be done after words in parens in TypeScript):
 Execute(Completion should not be done after parens in TypeScript):
   AssertEqual '', ale#completion#GetPrefix(&filetype, 3, 15)
 
+Execute(Completion prefixes should work for other filetypes):
+  AssertEqual 'ab', ale#completion#GetPrefix('xxxyyyzzz', 3, 14)
+
 Execute(ale#completion#Show() should remember the omnifunc setting and replace it):
   let &l:omnifunc = 'FooBar'