Improve TikZ backend

Use a more efficient string concatenation method based on
http://www.skymind.com/~ocrow/python_string/.

Close #47.
Merge pull request #119 from r-gaia-cs/tikz-str-as-list

Implemented improvement at TikZ backend
Implement the column setting option for yaml
Add column information on file-format.rst and change to col_* instead of col-*