Let dashing respond to signals (i.e. SIGTERM)

Above all this makes docker stop work correctly.
Supply custom widget IDs upon docker run.

Use syntax:
  docker run -P frvi/dashing <GIST_ID> <GIST_ID> <GIST_ID> ...

This change sets the entrypoint in the dockerfile which makes
it impossible to get a shell in the image by just supplying
/bin/bash as an argument to run. Use the --entrypoint option
instead (see section "Debugging" in README.md).
Expose dashing configuration as a seperate volume.
Cleaning up README.md