mdoc v0.7.1

• #109 Fix #102
• #110 Publish binary version mdoc_2.12 instead of full vesrion mdoc_2.12.8
• #107 Upgrade to Scala 2.12.8

Breaking change

The mdoc module is now cross-publishing against the Scala binary version so the artifact name has changed from mdoc_2.12.7 to mdoc_2.12

- before
+ now
- "com.geirsson" % "mdoc" % "VERSION" cross CrossVersion.full
+ "com.geirsson" %% "mdoc" % "VERSION"