Fix help message for CLI

Fixes #895
Allow whitespace control on comments

This changes the call signature for the CommentNode constructor, which is a potentially breaking change for AST users.

Fixes #866
Add test for whitespace control w/ partial indent

Verifies the behaviors discussed in #858
Add preventIndent option for partials

This disables the standalone partial indent behavior required by the Mustache spec and allows for users to utilize partials in the same manner as under 1.x.

Fixes #858
Simplify BlockNode by removing intermediate MustacheNode