enforced list data type for pymode_lint_ignore (gh issue 786)

doc/pymode.txt

@@ -309,10 +309,9 @@ Default code checkers (you could set several)          *'g:pymode_lint_checkers'
 Values may be chosen from: `pylint`, `pep8`, `mccabe`, `pep257`, `pyflakes`.
 
 Skip errors and warnings                                 *'g:pymode_lint_ignore'*
-E.g. "E501,W002", "E2,W" (Skip all Warnings and Errors that starts with E2) and
-etc
+E.g. ["W", "E2"] (Skip all Warnings and the Errors starting with E2) etc.
 >
-    let g:pymode_lint_ignore = "E501,W"
+    let g:pymode_lint_ignore = ["E501", "W",]
 
 Select some error or warnings.                           *'g:pymode_lint_select'*
 By example you disable all warnings starting from 'W', but want to see warning

pymode/lint.py

@@ -35,10 +35,12 @@ def code_check():
         # Fixed in v0.9.3: these two parameters may be passed as strings.
         # DEPRECATE: v:0.10.0: need to be set as lists.
         if isinstance(env.var('g:pymode_lint_ignore'), str):
+            raise ValueError ('g:pymode_lint_ignore should have a list type')
             ignore = env.var('g:pymode_lint_ignore').split(',')
         else:
             ignore = env.var('g:pymode_lint_ignore')
         if isinstance(env.var('g:pymode_lint_select'), str):
+            raise ValueError ('g:pymode_lint_ignore should have a list type')
             select = env.var('g:pymode_lint_select').split(',')
         else:
             select = env.var('g:pymode_lint_select')