Feature/542 allow customistion of email from reply to

As per #542, this should add the reply-to customisation. 

Re the need for "From name", reading through the docs/code for the Mail gem, it seems to support the convention of "Fred Example &lt;fred@example.com&gt;" and should suffice for the requirement for adding a name to the outgoing email. It'd just be a matter of adding this in the config in this format.

I'd suggest this is more in line with how emails are constructed. I haven't tested the Mail gem to make sure it behaves as expected. If not, then it'd be a matter of inserting the headers within prepare_mail method 
`@smtp_config` should probably be treated as invariant here (copying rather than altering loaded config data) but that's not really explicit in the code itself. Slight change to:

```ruby
m_reply_to = @smtp_config ? (@smtp_config['reply_to'] || m_from) : "flapjack@#{@fqdn}"
```