Read markdown files with plugin encoding setting

Default codec used by 'scala.io.Source.fromFile' depends on system environment that could have other encoding. For example, Windows with cyrillic locale has windows-1251 charset. But lwm:transate raises java.nio.charset.UnmappableCharacterException during reading markdown file in another encoding (utf-8).

To eliminate this error lwm need to read markdown files with explicit encoding from lwm plugin settings.

P.S.
I state that this tiny one-liner patch is my original work and I provide it for grizzled-scala project under [BSD License](http://software.clapper.org/sbt-lwm/license.html). :)