Reference multiple issues in the fragment filename

[SmileyChris]

Previously discussed in #348

Currently, you can have a news item with multiple issues by making multiple fragments with the same content. For example:

towncrier create --content "An important ossue" 123.bugfix
towncrier create --content "An important ossue" 125.bugfix

Resulting in:
...
- An important ossue (123, 125)

This is a bit ugly, especially if you want to update the text of the issue before a towncrier create since you have to do it in each file.

A suggestion from @hynek was that you could have the issues comma seperated in the fragment filename. For example:

towncrier create --content "An important issue" 123,125.bugfix