Added Tableize to wrap content into a table.

- Added :Tableize which accepts a range. It checks if the content has a
  delimiter defined by g:table_mode_delimiter and if so, it adds a table
  around it.
- Added <Leader>T mapping, which is defined by g:table_mode_tableize_map
  that applies :Tableize on the visually selected range.
- Added options for tableize.
- Updated vim doc.
Fixed ConvertDelimiterToSeparator

- We now aggressively add separators to a line even if it does not have
  the g:table_mode_delimiter. If the user calls Tableize, we should
  trust the user he wishes to tableize everything. Earlier
  implementation skips & fails for lines in text that did not have the
  g:table_mode_delimiter and hence formed inconsistent tables.

  Everything can be undone with a single 'u' anyways.