Feat: --migrate-theme command

[rochacbruno]

Assuming a running blog, where the user already have the /templates and /static folders generated when the user first executed marmite to start a new site.

scenario

User created the site using and the marmite inputdir --init-site command.
inputdir/templates and inputdir/static was created by marmite.

Now the user wants to upgrade to marmite 0.2.6, which brings a lot of improvements to the templates, how does the user get their templates and static files updated?

marmite inputdir --migrate-theme  # migrate from default to named theme

This command will:

1. Create a new folder inputdir/theme_{date} (where date is current date + time as 2015_01_01_23_42_55, eg. theme_2015_01_01_23_42_55)
2. Copy the embedded templates from example/templates (initialize_templates) and embedded static from example/static (generate_static) to inputdir/theme_{date}, so the inputdir/theme_{date} will have templates and static subfolders taken from marmite embedded template.
3. Copy from user /templates and /static any files that are not already present on theme/templates and theme_{date}/static
4. Generate inputdir/theme.json
5. Set theme: theme_{date} on marmite.yaml

Done.

Now the site will be built from the new "theme_{date}" and the legacy templates and static are kept in case user needs to move additional custom files.

---

This provides a smooth way to upgrade from legacy built-in theme to a named theme but keep using the same templates and style.

---