Add support for absolute paths in envFileName

[ruurtjan]

Sbt-dotenv currently always prepends the envFileName with the base directory.

sbt-dotenv/src/main/scala/au/com/onegeek/sbtdotenv/SbtDotenv.scala

Line 89 in ab634f7

val dotEnvFile: File = new File(s"${baseDirectory}/${fileName}")

I think it would make sense to check if the first character of envFileName is a /, and if so, don't prepend the base directory.

The use case for me would be to inject an envfile in GitLab CI. It allows you to define environment specific files, and provides an absolute path to these files in an environment variable. It would be convenient to be able to plug that into sbt cli like so:
sbt "set ThisBuild / envFileName := \"$SBT_DOT_ENV\"" test

Would you accept a PR for this?

[Philippus]

Sure, sounds like a welcome addition.

[Philippus]

Fixed in #139.