Theming support

[KrishnaPG]

I see that meteor-starter is already using LESS, which (I guess) makes it easy for 'theme switching' for boostrap UI frontend.

Since this is a startup helper template users build on, it would be great if users are allowed complete customization of their frontend (in a single place).

Towards that purpose, requesting to consider the below:

1. bootstrap-variables.less presently have most of the less configurations commented out - may be they should all be uncommented and exposed for user
2. Presently only one primary color brand-primary is being used - please consider adding brand-secondary color option also.
3. Not sure how they do it - but some of the bootstrap themes out there has this nice Change Skin option that dynamically changes the complete UI colors (on the fly). It would be a gread value-add to have that (to build demo sites and all).