Integrate markdown-it-include plugin

[gnaeus]

Ability to include content of one markdown file to another for building single PDF from multiple markdown source files. Syntax: :[alternate-text](relative-path-to-file.md). See #86

Example

├── [plugins]
│   └── README.md
├── CHANGELOG.md
└── README.md

INPUT

Content of README.md
:[Plugins](./plugins/README.md)
:[Changelog](CHANGELOG.md)

OUTPUT

Content of README.md
Content of plugins/README.md
Content of CHANGELOG.md

[yzane]

Thanks for your PR!