Extension dev

[tomlin7]

• For now, a hello biscuit template has been added to github
• Later, this will be cloned when biscuit ext new ... command is used
• Maybe when there are more templates (language, theme, etc) we can go with biscuit ext new --<template> ...
• Extension dev runs -- something like biscuit ext dev will launch a biscuit instance with that extension on
• Extension tests -- I have added some tests to template repo demonstrating how it's done, so they can just run pytest and it will work

[tomlin7]

update: we may use cookiecutter for this