[2.0 Proposition] Styled help

[AdrieanKhisbe]

We should add the option to have styled help section, mainly highlight the commands and the different sections for better readability.

This could be done easily with chalk using bold&dim. (and eventually colors) With 2.0 this could be done as default, (with an eventual possibility to opt out this behavior)

cc https://github.com/dthree/vorpal/pull/272