Extract out gem related code to Pry::Rubygem

And fix some typos, and prettify some lines.

Signed-off-by: Kyrylo Silin <kyrylosilin@gmail.com>
Implement `Rubygem::install`

Use the code of `gem-install` command as a basis.

Signed-off-by: Kyrylo Silin <kyrylosilin@gmail.com>