Introduces a new command line switch `--must-staple` to `run` and `renew`.
Using this switch will add the must staple TLS extension to the CSR generated by lego and thus also to the generated certificate.
This does not work with user specified CSRs!
Fixes#270
Even though the std lib does the heavy lifting, this should verify (somewhat) that the helper functions are working.
Better tests would involve replacing crypto/rand.Reader with a non-random Reader then comparing the contents to make sure the proper std lib crypto functions were called and their values returned.
