Add some tools to ftplugin and a couple of fixes.

- Add vimpeg#peg#quick_test(lines) to allow parsing a commented out
  (with '; ') text sample in the vimpeg grammar file.
- Add :VimPEGTest to print the output of quick_test().
- Add a mapping for :VimPEGTest (<leader><leader>).
- Better detection of Tabular.
- Fix a couple of problems in peg.vim.