use 'detect_on_enter' instead of 'auto' and document the option

With posva#128, I can imagine some people don't want vim-vue to scan for new
languages on each text change event. So instead of only having an 'auto'
option, users will be able to choose between 'detect_on_enter' and
'detect_on_change'.

Author: Adriaan Zonnenberg

readme.md

@@ -130,3 +130,9 @@ let g:vue_pre_processors = []
 ```
 
 Available pre-processors are: coffee, haml, handlebars, less, pug, sass, scss, slm, stylus, typescript
+
+When `g:vue_pre_processors` is set to 'detect_on_enter' instead of a list, vim-vue will detect the pre-processors used when a file is opened, and load only their syntax files.
+
+```vim
+let g:vue_pre_processors = 'detect_on_enter'
+```

syntax/vue.vim

@@ -31,7 +31,7 @@ function! s:should_register(language, start_pattern)
   if exists('g:vue_pre_processors')
     if type(g:vue_pre_processors) == v:t_list
       return index(g:vue_pre_processors, s:language.name) != -1
-    elseif g:vue_pre_processors is# 'auto'
+    elseif g:vue_pre_processors is# 'detect_on_enter'
       return search(a:start_pattern, 'n') != 0
     endif
   endif