Custom class prefix

Hi, we've found partially baked feature we badly needed.

Currently [bootstrap.json](https://github.com/sapegin/grunt-webfont/blob/v0.2.0/tasks/templates/bootstrap.json) (and BEM) has `classPrefix`, but it's not available nor used inside [bootstrap.css](https://github.com/sapegin/grunt-webfont/blob/v0.2.0/tasks/templates/bootstrap.css). Although, it's already being used in [demo HTML template](https://github.com/sapegin/grunt-webfont/blob/v0.2.0/tasks/templates/demo.html#L72)

This commit changes following:
- extends CSS context with vars from JSON e.g. `<%= baseClass %>`, `<%= classPrefix %>`
- replaces `icon_` with `<%= classPrefix %>` in BEM templates
- replaces `icon-` with `<%= classPrefix %>` in Bootstrap templates
- replaces `icon-` with `<%= _.dasherize(classPrefix) %>` in BEM templates

Sorry, but no tests yet.