Previously, the paths were just relative paths and depended on the working
directory of the sbt process. However, that's unreliable when the akka
sources are included in another build as in the akka-http nightlies.
* Add CopyrightHeader support for sbt-boilerplate plugin.
* Add CopyrightHeader support for `*.proto` files.
* Add regex match for both `–` and `-` for CopyrightHeader.
* Add CopyrightHeader support for sbt build files.
* Update copyright from 2018 to 2019.