Mulled Support - Part 3

Stuff that will probably not be done before GCC 2017 - but should still be done and added to docs.

- [ ] Allow local recipes when using ``planemo mulled``.
- [ ] Work through fleeqtk based example for documentation.
- [ ] Singularity all the stuff.
- [ ] Implement Singularity Support in Planemo.
- [ ] Update planemo-machine to pre-install and pre-configure Singularity.
- [ ] Update docs to walk through a mulled example using Singularity.
- [ ] Allow combining tests for multi-requirement containers in mulled toolkit.
- [ ] Enable test in multi-package-containers.
- [ ] Update multi-package-containers to allow publication of single recipe containers not covered by blanket bioconda publishing.
- [ ] Provide some best-practice guidelines for triggering new builds of packages.

Start by breaking the checklist into separate efforts around the `planemo mulled` command, `planemo-machine`, Singularity, and `multi-package-containers`. Read the existing mulled documentation and related command behavior first; completion would require implementing the selected item and updating or adding the requested walkthroughs, tests, or publication guidance.

Written by the indexing model from the issue text.