Support AsciiDoc syntax in Jira issue descriptions and comments

AsciiDoc has become a very popular format for writing documentation, README files, website contents and even books in the open source community. It's well supported by tool chains for producing documentation and platforms such as GitHub which e.g. render AsciiDoc-formatted files.

It'd therefore be very useful if AsciiDoc could also be used when authoring Jira issues, e.g. in the description field and comments. That'd mean less friction when switching between different tools, as e.g. the syntax for hyperlinks differs between Jira and AsciiDoc.