feat(gen): add base tag support for index.html

To be able flexible configure resources base tag, depending on actual deployment folder.

I use to deploy client side application directly to github via gh-page. If application use client-side routing or accessing resources by relative path that application would not work there.

Added grunt-targethtml task to configure the base html tag.

Examples, 

`http://example.com` => base: '/';
`http://example.com/dashboard` => base: '/dashboard/';

using that would make app work in any virtual directory.