Support relative paths for defaults-file

- introduce mutator pattern for flag values
  - add make_path_absolute mutator
    - add tests
- refactor DEFAULT_FLAGS constant to PERCONA_FLAGS, which can contain additional metadata
  - add tests
- bump version to 0.0.3
  - update CHANGELOG
- update README
Merge pull request #2 from steverice/support-relative-paths

Support relative paths for defaults-file