`flet create` CLI

[FeodorFitsner]

flet create creates a new Flet app from a template.

Template has at least assets directory created (related to #1043) with favicon.png and manifest.json inside.

flet create parameters:

• --project-name the name of the project to put into manifest.json.
• --description project description to put into manifest.json.
• --template the name of the template to use for new project (all templates are in https://github.com/flet-dev/templates).

[ndonkoHenri]

Parameter ideas:

• flag for Appveyor stuffs (appveyor.yml)
• another for fly.io stuffs (Dockerfile,..)

Templates should also have a requirements.txt file with flet.