Allow multiple and formatting-agnostic template files

[vy]

Exporting should not enforce anything on the used formatting; AsciiDoc, Markdown, etc. Users should be able to export using .release-notes.adoc.ftl, .release-notes.txt.ftl, etc. next to each other. Note that once the formatting restriction is lifted, XSD needs to remove format="asciidoc" restriction too.

Goals

1. Remove any restrictions on the template file
2. Support multiple template files
3. Support optional template files (don't fail the build if the template is not found)
4. Support using the version in the target changelog template (e.g., .release-notes.md.ftl to %v.md, where %v will be substituted with the associated release version)