Make email options of find_interesting_reviews more flexible.

This enables a few requested improvements on the original review of this
script at https://reviews.llvm.org/D46192.

This introduces 2 new command line options:

* --email-report: This option enables specifying who to email the generated
  report to. This also enables not sending any email and only printing out
  the report on stdout by not specifying this option on the command line.
* --sender: this allows specifying the email address that will be used in
  the "From" email header.

I believe that with these options the script starts having the basic
features needed to run it well on a regular basis for a group of
developers.

Differential Revision: https://reviews.llvm.org/D47930

llvm-svn: 335948