Fix #257 in preparation for #427, standardise options with fallbacks, and make it so every value can be computed dynamically

ale_linters/sml/smlnj.vim

@@ -1,12 +1,6 @@
 " Author: Paulo Alem    <paulo.alem@gmail.com>
 " Description: Rudimentary SML checking with smlnj compiler
 
-if exists('g:loaded_ale_sml_smlnj_checker')
-    finish
-endif
-
-let g:loaded_ale_sml_smlnj_checker = 1
-
 function! ale_linters#sml#smlnj#Handle(buffer, lines) abort
     " Try to match basic sml errors