Tig-like command line arguments for startup mode

**Is your feature request related to a problem? Please describe.**
Sometimes I want to look at something specific, like a file blame or a branch view. Currently this means starting `gitui` and navigating to that state. This is redundant, especially when having to navigate to a file and open the blame view, instead of opening a blame from the terminal directly.

**Describe the solution you'd like**
Some tig-like cli arguments:
- `blame ` to directly open gitui in a file blame view
- `` to start with a log view at that refspec
- `stash` to start with the stashes view

**Describe alternatives you've considered**
Manually doing the navigation after opening `gitui`.