- Renamed option `headSubject` to `headTagName` because the option
  involves a tag name instead of subject.

- Renamed option `nextTagDate` to `headTagDate`.

- Option ~~headSubject~~ headTagName can't be set to an existing tag
  name anymore. Having duplicate tag names is ambiguous.

- Reference to HEAD revision in Gitchangelog::gitTags changed from
  'HEAD' to ''. Any non empty value could result in duplicate tag names.

- Changed shell_exec function calls to exec function call where
  multiline output is expected. This avoid the overhead of exploding the
  single output string of shell_exec.

- GitChangelog::$setToTag now refers to head revision of repository with
  value null instead of empty string.

- Fixed thrown exceptions at docBlocks.

Closes #9
- The html and markdown renderer, convert issues ids (like #7) into a
  hyperlink which is set as class property.
- The html and markdown renderer, convert hashes (like 2587485) into a
  hyperlink which is set as class property.

- Header 1 tag inside Paragraph tag when there's no commit data.
- Issues are always formatted, even with no issueUrl specified.
- Hashes are formatted while no commitUrl is specified.

- Removed default value of MarkDown::$issueUrl.
- New line prepended to changelog instead of separating the header and
  message when there's no commit data.
- Message has trailing space when there are no subjects.
- Hashes are formatted while no commitUrl is specified.

- GitChangeLogTest::testFetchTagsCached expects wrong value.
- Changed visibility of GitChangeLogTest::setPrivateProperty().
- Added typehints for parameters of
  GitChangeLogTest::setPrivateProperty().
- Added typehints for parameters of
  GitChangeLogTest::getPrivateProperty().
- Removed GitChangeLogTest::testBuildAscendingCommitOrder().
- Removed GitChangeLogTest::testBuildDescendingCommitOrder().
- GitChangeLogTest::testRemoveLabel() tests without default labels.
- Add test for removing a label that doesn't exist.

- Unreachable lines taken out from code coverage.

Instead of defining a path to a file which content is used to append the
generated changelog, the generated changelog can now also be appended by
any string value.

Remove redundant comments.

Updated to content to reflect changes since commit a6f72c6.