Tag prefix should be customizable

[JavierSegoviaCordoba]

Describe the bug
Currently, the plugin has hard coded the v when comparing version tags, which is not correct as the prefix can be custom.

gradle-changelog-plugin/src/main/kotlin/org/jetbrains/changelog/ChangelogPlugin.kt

Lines 86 to 88 in d703c90

	previousVersion == null -> "/commits/v$currentVersion"
	else -> "/compare/v$previousVersion...v$currentVersion"

Expected behavior
Allow setting the prefix via versionPrefix property in order to set the comparison.