Add `s:GetXterm2rgbTable` to lazily setup the xterm2rgb table

Ref: https://github.com/lilydjwg/colorizer/issues/12
Use Vim's `pow` function, instead of internal one

The internal one only worked for n<=2, but got used for n=2 anyway.
It showed up quite high in the performance profile though, let alone
because of the number of calls/invocations.
Skip calling matchend() when breaking anyway
Merge remote-tracking branch 'blueyed/performance-fixes'

Conflicts:
	plugin/colorizer.vim
move function definitions to autoload
use TextChanged event when possible