Extract class in Pull

All subcommands get their own class.
Files for not yet implemented subcommands are also setup to avoid
breakage.
Add first aliases for pull command(s)
Add diff to Pull::Show
Add patch to Pull::Show
Delete new truncate method in Formatting

That functionality is already implemented in the same module
Format diff output for Pull::Show
Format patch output in Pull::Show and refactor
Add FIXME note to Formatting

Diff output shows minor inconsistencies
Improve help for Pull
Add fetch command for Pull
Add pull fetch alias
Shift order of Pull subcommands in help
Add TODO note about proper handling of pr fetching
Move aliases to own class

- Aliases is now an own command, which displays available aliases
- Move alias logic from GHI to Aliases