Feature/sendmail

[renozao]

Hi,

this PR is to enable users to define and use their own sendmail function as a hook.
I had to use this because of some restrictions on our clusters. I have a workaround in place that I can use to sen emails, but I needed this simple hook to trigger it, instead of the default SMTP-based function from sendmailR.

It has no impact on the rest of the package. The custom mailing function only needs to have arguments status, from, to, subject, body.

[coveralls]

Coverage decreased (-0.1%) to 56.46% when pulling dcd0e96 on renozao:feature/sendmail into b784e86 on tudo-r:master.

[coveralls]

Coverage decreased (-0.1%) to 56.46% when pulling 86a1338 on renozao:feature/sendmail into b784e86 on tudo-r:master.

[renozao]

Please tell me what I need to do to make this pull request going through.
Thanks