Check 'rtp' when needed instead of onload

Before this commit, SnipMate would check 'rtp' only when it's loaded.
Any changes to 'rtp', say at runtime, would not affect SnipMate's
snippet searching procedures.

See #247.

autoload/snipMate.vim

@@ -384,7 +384,7 @@ function! snipMate#DefaultPool(scopes, trigger, result) abort
 		let s:lookup_state.extends = []
 
 		for expr in s:snippet_filenames(scope, escape(a:trigger, "*[]?{}`'$|#%"))
-			for path in g:snipMate.snippet_dirs
+			for path in s:snippet_dirs()
 				for file in s:Glob(path, expr)
 					source `=file`
 				endfor
@@ -416,6 +416,10 @@ fun! snipMate#GetSnippets(scopes, trigger) abort
 	return result
 endf
 
+function! s:snippet_dirs() abort
+	return get(g:snipMate, 'snippet_dirs', split(&rtp, ','))
+endfunction
+
 function! snipMate#OpenSnippetFiles() abort
 	let files = []
 	let scopes_done = []
@@ -425,7 +429,7 @@ function! snipMate#OpenSnippetFiles() abort
 		let files += s:snippet_filenames(scope, '')
 	endfor
 	call filter(files, "v:val !~# '\\*'")
-	for path in g:snipMate.snippet_dirs
+	for path in s:snippet_dirs()
 		let fullpaths = map(copy(files), 'printf("%s/%s", path, v:val)')
 		let exists += filter(copy(fullpaths), 'filereadable(v:val)')
 		let notexists += map(filter(copy(fullpaths),

plugin/snipMate.vim

@@ -93,8 +93,7 @@ endif
 let g:snipMate['get_snippets'] = get(g:snipMate, 'get_snippets', funcref#Function("snipMate#GetSnippets"))
 
 " List of paths where snippets/ dirs are located
-let g:snipMate['snippet_dirs'] = get(g:snipMate, 'snippet_dirs', split(&rtp, ','))
-if type(g:snipMate['snippet_dirs']) != type([])
+if exists('g:snipMate.snippet_dirs') && type(g:snipMate['snippet_dirs']) != type([])
 	echohl WarningMsg
 	echom "g:snipMate['snippet_dirs'] must be a List"
 	echohl None