Extend the CLI to use custom templates

[raksiv]

Users will want to experiment with their own templates, it should be possible to do this without officially releasing a template.

Add a configuration in the CLI to let them configure their template sources

• Add notes to readme to describe how they would do this.

[tjholm]

A template source flag might work well for this (many other CLIs implement it this way).

nitric new --template git://github.com/myorg/mytemplates/mytemplate