convert (s *session) writeCommandPackage:
convert (s *session) writeCommandPackage:

- add createMapFile flag to Options
- set it to true when running main
- create mapfile only if createMapFile is true (is so when running main, like old behaviour)
- create codefile only if Options.Target is not set (is so when running main, like old behaviour)
- set codefile to Options.Target (an io.Writer) and mapfile to Options.SourceMap
- use Options.Target and Options.SourceMap instead of codefile and mapfile
  this allows Options.Target and Options.SourceMap to be set to another io.Writer
  when package is imported as library
- set the MappingCallback only if Options.SourceMap is set. Allows to skip
  the source map in library mode

jargv/gopherjs,roth1002/gopherjs,gopherjs/gopherjs,rasata/gopherjs,flimzy/gopherjs,bpowers/gopherjs,ebroder/gopherjs,Yanjing123/gopherjs,rasata/gopherjs,Yanjing123/gopherjs,jargv/gopherjs,Archs/gopherjs,roth1002/gopherjs,jeremyschlatter/gopherjs,Archs/gopherjs,gopherjs/gopherjs,albrow/gopherjs,nmfzone/gopherjs,gopherjs/gopherjs,bpowers/gopherjs,nmfzone/gopherjs,bpowers/gopherjs,gopherjs/gopherjs,govlas/gopherjs,roth1002/gopherjs,ebroder/gopherjs,Archs/gopherjs,flimzy/gopherjs,govlas/gopherjs,albrow/gopherjs,albrow/gopherjs,jeremyschlatter/gopherjs,jargv/gopherjs,gopherjs/gopherjs,govlas/gopherjs,Yanjing123/gopherjs,nmfzone/gopherjs,jeremyschlatter/gopherjs,ebroder/gopherjs,flimzy/gopherjs,rasata/gopherjs