Stylistic refactor in Formatting
Delete obsolete blk params in Pull::Merge#options

Switch options do not need them.
Always return diff in format_diff

gsub returns nil when there was no substition
Avoid lurking bug - don't use + with strings

... when you cannot guarantee that they are not nil...
Move show_pull_request to Pull

The parent class makes good use of it, as there are several subclasses
that might call it
Unify interfaces and logic in Pull::Close and Edit
Unify Pull::Create and document what's happening

There are some gotchas here - check the comments!
Use heredoc in Pull::Merge

And document a gotcha about whitespace.
Add TODO about messaging in Pull::Merge
Add comments about the prohibition of empty bodies

It is possible to create a pull request without a description body.
That's a bad habit though - don't allow it!