Some small changes

Merged into one PR as they all will produce merge conflicts:

- Purged out constants from classes and lowercased TERM_24BIT option as it is not a constant
- Added support for italic and underline attributes in shell renderer
- Added a way to specify renderer options on command-line
- Added tmux_escape option

Note: the latter may not fix #199 because it may produces gap in place of a border character:

![Gap between ... and directory name](http://img-fotki.yandex.ru/get/6439/9151298.2/0_93b4c_5d9a317f_orig.png)