feat: add configuration files

Permit *git-changelog* be configured permanently via config files.

Resolves: pawamoy#54.

docs/usage.md

@@ -402,6 +402,67 @@ Other options can be used to help *git-changelog* retrieving
 the latest entry from your changelog: `--version-regex`
 and `--marker-line`.
 
+
+## Configuration files
+
+Project-wise, permanent configuration of *git-changelog* is possible.
+By default, *git-changelog* will search for the existence a suitable configuration
+in the ``pyproject.toml`` file or otherwise, the following configuration files 
+in this particular order:
+    * ``.git-changelog.toml``
+    * ``config/git-changelog.toml``
+    * ``.config/git-changelog.toml``
+    * ``~/.config/git-changelog.toml``
+
+The use of a configuration file can be disabled or overridden with the ``--config-file``
+option.
+To disable the configuration file, pass ``no``, ``None``, ``false``, or ``0``:
+
+```bash
+git-changelog --config-file no
+```
+
+To override the configuration file, pass the path to the new file:
+
+```bash
+git-changelog --config-file $HOME/.custom-git-changelog-config
+```
+
+The configuration file must be written in TOML language, and may take values
+for most of the command line options:
+
+```toml
+bump-latest = false
+convention = 'basic'
+in-place = false
+marker-line = '<!-- insertion marker -->'
+output = 'output.log'
+parse-refs = false
+parse-trailers = false
+repository = '.'
+sections = ''
+template = 'angular'
+version-regex = '^## \[(?P<version>v?[^\]]+)'
+```
+
+In the case of configuring *git-changelog* within ``pyproject.toml``, these
+settings must be found in the appropriate section:
+
+```toml
+[tool.git-changelog]
+bump-latest = false
+convention = 'conventional'
+in-place = false
+marker-line = '<!-- insertion marker -->'
+output = 'output.log'
+parse-refs = false
+parse-trailers = false
+repository = '.'
+sections = ''
+template = 'keepachangelog'
+version-regex = '^## \[(?P<version>v?[^\]]+)'
+```
+
 [keepachangelog]: https://keepachangelog.com/en/1.0.0/
 [conventional-commit]: https://www.conventionalcommits.org/en/v1.0.0-beta.4/
 [jinja]: https://jinja.palletsprojects.com/en/3.1.x/