Release 0.3.1

Prepare for 0.3.0 (#13)

Prepare for 0.2.1 (#8)

* Summary

- Simplify configuration and group by tag titles ([#2](#2))
- Improve configuration file lookup and support post-processors ([#3](#3))

** Features

- Simplify configuration by omitting tags, defaults, etc. All we
    need now are simple keyword->title mapping and keywords without
    titles are skipped from the report. The aggregation is now done
    on title (instead of tag keyword) which allows us to group
    multiple tags into one report section. For example, with tags
    `break => Breaking Change` and `braek => Breaking Change` we
    can accommodate simple typos.

- The tool now automatically uses a configuration file
    `.changelog.yml` in the current directory. If a file with that
    name doesn't exist in the current directory, it looks up the
    directory tree in the parent directories as long as it reaches
    the root. This simplifies project specific configuration by
    checking in a `.changelog.yml` in the repo root.

- The configuration can now specifiy post-processors
    that look for specific markers like (e.g. `JIRA-12345`) and
    replaces them with suitable replacements. This makes adding
    links like [JIRA-1234](http://jira.company.com/view/JIRA-1234)
    easy. In general, a post-processor is a `(lookup,replacement)`
    tuple where a `lookup` is a regex with named capture groups
    like `JIRA-(?P<ticket>\\d+)`) and
    [replacement](https://doc.rust-lang.org/regex/regex/index.html#example-replacement-with-named-capture-groups)
    is a simple string that refers to the named capture groups like
    `[JIRA-$ticket](http://jira.company.com/view/JIRA-$ticket)`

** Breaking Changes

- Configuration format has changed and old configuration
    files will need some tweaks.

Increment version