Add --base flag to gulp build to allow setting base href location

[MichaelMarkieta]

• I understand that GitHub issues are not for tech support, but for questions specific to this generator, bug reports, and feature requests.

It would be great to have a gulp build flag that lets use set the base href. This will help out users who deploy to virtualhosts/subcontext directories on their webservers.

[MichaelMarkieta]

Looks like angular-seed has this, although wrong angular version: mgechev/angular-seed#1575 Will look into this some more later.