Blame and stash start options

[alexmaco]

This Pull Request partly implements #1382

It changes the following:

• preamble refactor and simplification of cli parsing
• parses start modes as subcommands
• starts with stash view or file blame view

I followed the checklist:

• [ ] I added unittests
• [x] I ran make check without errors
• [x] I tested the overall application
• [ ] I added an appropriate item to the changelog