Added mixed mode highlighting for best of both worlds speed improvement

[Redsandro]

Fixed issue #9

Added "mixed" mode for docs_render for when you are working with both small and large documents. This will color-code (AKA highlight) documents smaller than docs_render_limit, and leave the larger ones as "plain".

[Redsandro]

This is so sweet, I recommend setting "mixed" as default. Optionally raise the limit a bit.

[myurasov]

Merged into https://github.com/myurasov/rockmongo.