skeleton templates for quickstart

[nikhedonia]

Currently buckaroo quickstart is hard-coded to create the project in a specific way.
Although we have been pushing Buckaroo as a c++ package manager, we actually support any language that buck supports.

We would love to have more skeletons for more languages.

So as a first step, extend quickstart to support a optional language parameter eg. buckaroo quickstart cpp.
Then make quickstart-skeletons live in ~/.buckaroo/quickstart/$lang folder.