Refactor output of an issue's comment in Show

Prepare addition of pull command
Prepare Formatter for Pull

Adds formatting methods for upcoming pull show command
Start Pull command with show subcommand
Adds options to Pull#show
Implement
Add display of commits for pull requests
Truncate overly long messages in pull show -c
Delete pull show --files option

Doesn't even exist in the API
Start help screen for Pull
Show head and base branch in Pull#show
Abort with invalid subcommands in Pull
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