make behaviour configurable so that `previousStableVersion` can ignore M0/RC0 versions

[pjfanning]

• sbt-dynver derives previousStableVersion (by default) from git tags (tags starting with v)
• it would be nice to make this optionally configurable so that users could set it up so that only v1.2.3 style tags are considered
• tags like v1.2.3-M0 and v1.2.3-RC0 could be ignored

sbt-dynver/sbtdynver/src/main/scala/sbtdynver/DynVerPlugin.scala

Line 44 in 52050ce

dynverGitPreviousStableVersion := dynverInstance.value.getGitPreviousStableTag,