New arguments for theme options (code-font, UI colorscheme)

[niloch]

This PR introduces 2 new jupyter-themer optional arguments for the overall UI theme (based on
https://github.com/powerpak/jupyter-dark-theme) and an argument for changing the font for code cells with one supported font Ubuntu_Mono.

For google web fonts to work properly the @import statements must be first before any other CSS styling. Therefore, it is necessary to check if there are font changes first and handle the necessary @import statements first before applying other CSS. To accomplish this, the @import is separated into a separate file for each font ending ing .import and the argument parsing is reordered to handle this case.

The custom_css_dir was incorrect in Python3 and the path was changed to work for both Python2.7 and Python3 by importing notebook instead of jupyter

Finally, slight adjustments were made to the padding in the wide layout and the base16-dark color scheme.
all changes can be seen by using the command

jupyter-themer -c base16-dark -l wide -b dark -f Ubuntu_Mono