Moved variables outside of the `mail` function

This seems uneccessary, but will mainly help avoid merge conflicts in future changes.

- `$subject` is defined elsewhere (but immediately set to `$SUBJECT`)
- `$msg` renamed to `$message`. I'm not poor; I can afford 4 extra characters
- `$headers` is a separate variable rather than passed directly as a string
Added `get_post_field`

Currently only used in one place, but will be used more in the future.
Added and applied `filter_name` and `filter_email`

The header is now properly cleaned to avoid at least one form of email injection (even in case the blog owner accidentally puts a bad email address in the `$EMAIL_ADDRESS` variable.

Both functions taken from http://mattgeri.com/blog/2012/01/escaping-input-to-the-php-mail-function/ (should I include attribution in the code as well?)
Added commenter name to mail header
Added "Reply to" field to mail header

This extra field will only be added if the user has supplied an email address.

This fix allows blog owners to reply to the user's comment by replying to their email (if they included an email address in the comment) if they want a private reply rather than a reply as a comment on the blog.
Improved email title

The title of the email sent out by `commentsubmit.php` has been improved to "Comment from <name> on '<post title>'" making it easier to sort comments when they get to your inbox.
Merge branch 'dev-email-title-additions'
Merge branch 'dev-reply-to-additions'

Conflicts:
	commentsubmit.php
Add function `get_post_data_as_yaml`

Made it a tad tidier, and will make the next step easier (see next commit).
YAML data is now attachment rather than email body

This has three benefits:
  [1] Rather than copy and paste the YAML data into a new text file, you can click the attachment and "save as" to where you want it.
  [2] You can easily see the comment in the body of the email unobscured by other YAML, allowing you to quickly sort spam from "real" comments.
  [3] For similar reasons as #2, you can eaily reply to the email. The one you are replying to won't see any YAML data, only their original comment.
Merge branch 'dev-attach-comment-to-email'
Add UTF-8 support in mail subject and body

Now umlauts and other foreign characters show up properly.

Resolves https://github.com/mpalmer/jekyll-static-comments/issues/9