Git-authorship

**Documentation**
- Update README with instructions for using mailmaps
- Update README with instructions for using `.git-blame-ignore-revs`
- Clarify path resolution differences for ignore revs vs. author-licenses/pseudonyms

Adds multiple features for cleaning up and normalizing authorship reports.

**Features**
- Common picture formats (`.png`, `.gif`, `.jpg`, and `.jpeg`) are omitted from authorship reports.
- `.git-blame-ignore-revs`, if present, is automatically considered when computing `git blame`
- Add `--ignore-revs-file` option to CLI to specify alternate ignore revs file location.
- Ensured consideration of a top-level `.mailmap` when computing authorship.

With this release, `git-authorship` once again has all the features it had in version `0.0.0`, but now much more usable and maintainable.

**Features**
- Restore `--pseudonyms` feature (previously removed in `0.1.0`)
- Add `--version` option to CLI
- Validate config files more strictly (i.e. provide more helpful error messages by failing early)
- Include descriptions of CSV config columns in CLI `--help` text

**Fixes**
- Output correct results when rerunning authorship with new configs on a previously analyzed revision.

**Maintenance**
- Any modifications to the raw `blame` results are now conducted in separate `augment`ation steps to improve maintainability.

**Features**
- Allow re-cloning the repository in `--no-cache` mode
- Allow saving reports to custom `--output` folder

**Fixes**
- Enable cloning into an existing, empty directory

**Maintenance**
- Improved performance of end-to-end tests

**Features**
- Restored ability to report `--author-licenses` (previously removed in `0.1.0`)

**Maintenance**
- Add testing instructions to CONTRIBUTING.md

**Documentation**
- Restore alt-text on PyPI