Use the following strategy to extract identifiers from syntax
highlighting:
- ignore match and region: they mostly contain arguments,
syntax groups and regular expressions;
- ignore "nextgroup=" if first word and subsequent arguments
"skipempty", "skipwhite", and "skipnl";
- ignore "contained" argument if first word;
- add remaining words to the list of identifiers.
Fix a bug where the word "match" was extracted while not being a keyword
of the syntax language.
vim.eval returns a str() object on py2, but our internal strings are all unicode().
We use vimsupport.VimExpressionToPythonType to wrap the conversion complexities.
