Fix #171 - Implement basic error highlighting

doc/ale.txt

@@ -322,6 +322,17 @@ g:ale_open_list                                               *g:ale_open_list*
   to `1`, in which case the window will be kept open until closed manually.
 
 
+g:ale_set_highlights                                     *g:ale_set_highlights*
+
+  Type: |Number|
+  Default: `has('syntax')`
+
+  When this option is set to `1`, highlights will be set in for erros and
+  warnings. The `ALEError` and `ALEWarning` highlight groups will be used to
+  provide highlights, and default to linking to `SpellBad` and `SpellCap`
+  respectively by default.
+
+
 g:ale_set_loclist                                           *g:ale_set_loclist*
 
   Type: |Number|