Bulker

- Major changes to the registry path treatment to accommodate tags
- Local/remote manifests are now loaded with `-f`
- Full registry paths are now required for load, run, and activate
- Config format includes a new hierarchical level for crate tags
- Add `host_commands` functionality (useful for `--strict` crates)

- Fix a bug with compatibility of load from registry for older pythons
(2.7/3.4).

- Allows `activate` to return the export command, to enable `bulker-activate`
shell approach (see tips)
- Implements initial bulker registry (file server version)
- Improved argument parsing for `bulker run`

- Add `bulker run` to execute a command in a crate
- `bulker init` will now guess `docker` or `singularity`
- `activate` and `run` commands now accept a comma-separated list of crates

- No longer require `-m` for `bulker load`
- Add support for singularity
- Allow choosing container engine with `bulker init -e`
- Add `-b` option to build/pull containers with `bulker load`

- First public bulker release