Add keyboard shortcuts to select next/prev color scheme

[objarni]

As a casual sublime user using the ColorSchemeSelector plugin, I'd like to be able to flip through the available color schemes of Sublime Text 2 without having to (re)learn how to set keyboard shortcuts in Sublime Text.

E.g. Ctrl+Sh+PgUp and -Dn seems free in my ST2 installation.

[gerardroche]

For ST3 try https://packagecontrol.io/packages/polyfill, it can allow you to navigate overlays and panels with ctrl+j and ctrl+k.