Read colors from os.environ

Default colors are not readable on some themed terminals. example:
![snapshot](https://cloud.githubusercontent.com/assets/767425/3826398/64d09120-1d61-11e4-8ada-31714ae9b564.png)

Environment variables named `tldr_blank_line`, `tldr_command_name`, `tldr_short_desc`, `tldr_example`, `tldr_command_args`, `tldr_other` can override default colors if exist. example:

`$ tldr_example="red on_blue bold underline" python tldr.py tar`