Contract templates and contract new from template using sc-meta

[popenta]

From now on, mxpy will use sc-meta for displaying the available contract templates and for creating a new contract using the available templates.

This PR contains a few breaking changes:

For the mxpy contract new command:

• the name argument is no longer a positional argument, should be used as a keyword argument: --name
• the --directory argument is no longer existent, --path should be used instead
• a new argument has been introduced, --tag. It is used to specify the framework version on which the contract should be created

For the mxpy templates command:

• the --tag argument has been added. It is used to specify the sc-meta framework version referred to