Fix #42 — Make it easy to specify custom templates by name

[AliSoftware]

This PR should fix #42.

Here's the current progress:

• Rename the --template option --templatePath to avoid confusion
• Make -t take a simple name now (defaulting to default), and use that name
• Manage case when -p and -t are specified together
• Make it look for templates into Application Support
• Create the template subcommand that lists available templates
• Create a Templates.md file that explains:
  • how templates work (link to Stencil doc)
  • the filters and nodes specific to SwiftGen (the ones not in Stencil core)
  • how to customize templates (where to save them, etc)