Add support for project templates

[javierluraschi]

RStudio project templates provide an easy way for users to create custom R projects. While developing an htmlwidget, I noticed this project could use a custom project template.

Here is how this works, creating a new project will list a new "R Package using HTMLWidgets"

In this example, we can create a project namedtestwidget:

Then that's it! A project gets created and running the widget produces a simple 1 2 3 custom <div/> element. I intentionally kept this widget simple to help users identify their components and customize with ease:

[jjallaire]

The only thing you might consider here is actually calling the htmlwidgets::createWidget function (which would keep you in sync with whatever the latest and greatest boilerplate is). This too generates a simple "hello, world" text widget. It also has a feature to download dependencies from Bower which I guess you could add to the wizard UI as properties.

[ramnathv]

@javierluraschi I love the idea and I agree with @jjallaire that it would be good to use createWidget and provide a field for widget dependencies (if we can make it searchable from bower, that would be super awesome!).

[javierluraschi]

Added a textbox for bower and making use of scaffoldWidget(). The textbox can't search for bower packages since, currently, RStudio templates have limited support of UI elements. Here is how the project page looks like with this change:

[javierluraschi]

@ramnathv anything else that we need in this PR? Thank you!

[ramnathv]

@javierluraschi I will take a look tonight and merge it. Thanks!

[javierluraschi]

@ramnathv could we merge? Each time that I create an htmlwidget, I find myself wanting this feature. Today is one of those days, thank you!

[ramnathv]

@javierluraschi I will review tomorrow and merge by Monday! Thanks.