Makes it possible to use Hatter to send mail.

ComposeCommand
  behaves like a command
    should respond to #execute
    should respond to #unexecute
  executing
    creates a temp file with the correct header
  after the user fills in the template
    correctly populates the mail with user input (FAILED - 1)

Failures:

  1) ComposeCommand after the user fills in the template correctly populates the mail with user input
     Failure/Error: @cmd.execute
     NoMethodError:
       undefined method `strip' for nil:NilClass
     # ./lib/commands/compose.rb:64:in `get_subject'
     # ./lib/commands/compose.rb:51:in `fill_in_mail'
     # ./lib/commands/compose.rb:18:in `execute'
     # ./spec/commands/compose_spec.rb:39:in `block (3 levels) in <top (required)>'