Add the ability to create mail merge fields inside Caracal documents

[danielrhodes]

Was recently in need of adding a mail merge field in document generated by Caracal. For those not familiar, mail merge fields are treated as a simple or complex field in OOXML. Even though mail merge fields (e.g. «=replace_me») are relatively straightforward in appearance, Word and libraries like Sablon do not recognize mail merge fields unless they are denoted as a field and treated in a special way.

This pull request adds the ability to create mail merge fields as simple fields and also auto-detects mail merge fields inside paragraph text, meaning you can use them in places like Caracal's tables without having to make any other changes.

Some examples:

doc.p do
    mail_merge '=replace_me'
end

doc.table [["Name", "Profession"], ["«=name»", "«=profession»"]] do
   ...
end

Also included some tests.

Things not done: did not implement more complex mail merge types, or nested types (i.e. a nested run). However, this could easily be done.

Looking for some feedback on how this was implemented and what it would take to have this merged in.