Add drag and drop config import closes #11004

As requested by @vincent3569 this PR allows user to import config.json settings via a drag and drop import. Fixes #11004.
Attached are some screenshots of the UI changes.

Drop target
![screen shot 2014-06-11 at 10 49 26](https://cloud.githubusercontent.com/assets/678372/3242850/56e89090-f150-11e3-8808-23f6bd7dd71b.jpg)

Error on uploading invalid file
![screen shot 2014-06-11 at 10 49 47](https://cloud.githubusercontent.com/assets/678372/3242852/58e3b532-f150-11e3-854c-3152b2db44b6.jpg)

Success message
![screen shot 2014-06-11 at 11 04 36](https://cloud.githubusercontent.com/assets/678372/3242853/5bcae874-f150-11e3-9063-153df42be53b.jpg)

Side nav
![screen shot 2014-06-11 at 11 04 53](https://cloud.githubusercontent.com/assets/678372/3242855/5ed4bb1c-f150-11e3-97bb-808aa150d4c1.jpg)

The customizer doesn't work with IE less than 9 or 10.
On Jun 13, 2014 1:48 PM, "Heinrich Fenkart" <notifications@github.com>
wrote:

> In docs/customize.html:
>
> > @@ -23,6 +23,18 @@
> >  <![endif]-->
> >
> >  <!-- Customizer form -->
> > +
> > +<div class="bs-docs-section">
> > +  <div id="import-drop-target" class="bs-dropzone">
> > +    <h2>✍</h2>
> > +    <p class="lead">Have an existing configuration? Upload your <code>config.json</code> to import it.</p>
> > +    <p>Drag and drop here, or <a id="import-manual-trigger" href="#" class="btn-link">manually upload</a>.</p>
>
> Okay, that seems reasonable. Maybe @cvrebert <https://github.com/cvrebert>
> can comment on IE < 9 support.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/twbs/bootstrap/pull/13790/files#r13745855>.
>