Add "componentsPath" option

[whyleee]

There is a common situation when the path to "bower_components" dir is "scripts/bower_components"or similar. And for some reason there is a code to strip leading path segment off, based on assumption that entire app will be one level down after "bower.json". But this is not true for a lot of projects (and moreover, this code doesn't work in Windows #39).

Supposing i have "scripts/bower_components" path and this script tag in html:
<script src="/scripts/bower_components/jquery/dist/jquery.js"></script>
it will be replaced by the next tag after build:
<script src="/scripts//ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
which is naturally wrong and should be:
<script src="//ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

Please do not depend on project structure, but add an option to set "bower_components" path manually. This is easy, because google-cdn already has this option.

[kynan]

This should also respect the cwd setting in .bowerrc, see #36