Chartpress

- Another ruamel.yaml type fix

- Add `--image-prefix` option
- Workaround ruamel.yaml bug when strings are all-digits
and start with 0 and contain an 8 or 9.
- Fix type checking for recent ruamel.yaml

- Fix image tagging when building multiple images
- Make image-building optional
- Show changes being made
- Support GITHUB_TOKEN env for pushing to gh-pages
- Include chartpress.yaml when resolving last changed ref
- Update only necessary fields

0.2.0-beta.1.003.asdf123

with --long
0.1.0-000.qwer123
0.1.0-002.sdfg234
0.2.0-beta.1.000.wert234
0.2.0-beta.1.003.asdf123


- Fix latest tagged commit
[66](https://github.com/jupyterhub/chartpress/pull/66)
([minrk](https://github.com/minrk))
- Fix bugs: index merge, image tag, g prefix, ignored tags
[64](https://github.com/jupyterhub/chartpress/pull/64)
([consideRatio](https://github.com/consideRatio))
- Support `valuesPath` pointing to a single `image:tag` string in addition to a
dict with separate `repository` and `tag` keys
[63](https://github.com/jupyterhub/chartpress/pull/63)
([minrk](https://github.com/minrk)).
- Support lists in `valuesPath` by using integer indices
[65](https://github.com/jupyterhub/chartpress/pull/65)
([minrk](https://github.com/minrk)), e.g. `section.list.1.image` for the
yaml:
yaml
section:
list:
- first: item
image: "not set"
- second: item
image: "image:tag" <--sets this here

- Add missing dependency on ruamel.yaml