Support using styles from Pygments plugins (#663)
Support using styles from Pygments plugins (#663)

`pygments.styles.STYLE_MAP` contains only styles built directly into
Pygments library. To list all available styles (including styles
registered by plugins), one should use `get_all_styles` generator.

For respective Pygments documentation, see:
http://pygments.org/docs/styles/#getting-a-list-of-available-styles
PKRoma/httpie,jakubroztocil/httpie,jkbrzt/httpie,jkbrzt/httpie,jakubroztocil/httpie,jkbrzt/httpie,jakubroztocil/httpie,PKRoma/httpie