add basic front-matters support

Cargo.lock

Cargo.toml

Makefile.toml

README.md

about.toml

examples/example.md

scripts/check.sh

scripts/create.sh

src/document/footer.rs

src/document/languages.rs

src/document/licenses.rs

src/processing.rs

src/processing/front_matters.rs

src/processing/front_matters/date.rs

src/processing/front_matters/document.rs

src/processing/front_matters/processing.rs

src/processing/front_matters/toml.rs

src/processing/front_matters/yaml.rs

src/processing/markdown.rs

src/processing/options.rs