Add ansi-enable and ansi-disable commands

[SeerLite]

To be used in custom hooks in user configuration:

hook global BufCreate '\*make\*' ansi-enable

[SeerLite]

I updated the README and also added ansi-disable.

@eraserhd What do you think about replacing render/clear commands with enable/disable commands? I can't see many uses for only render/clear but maybe I'm missing something

[eraserhd]

Regarding replacing render/clear with enable/disable... I'm not really sure.

I'm all for a simpler interface. Assuming ansi-enable renders the buffer and ansi-disable clears the highlighting for the buffer, here are a few thoughts:

• I'm ok with adding fifo hooks to non-fifo buffers.
• Do we ever want to disable then re-enable on a buffer? That would clear existing colors.

Ok, so I think I'd propose:

• ansi-enable calls ansi-render and adds hooks, so it works on static buffers too.
• ansi-disable removes hooks, does not clear colors.
• ansi-clear clears colors.
• We recommend people normally use ansi-enable.