introduce pegtl based parser::mediatype

[klokane]

It also replace JSON(Schema)Regex by MediaTypeMatcher

[tjanc]

Currently your grammar lives in src/parser. What about introducing a sub-project on root level, apib-parser, like this?

apib-parser
├── CMakeLists.txt
├── include
│   └── apib
│       └── grammar
│           └── mediatype.hpp
├── src
│   └── mediatype.cpp
└── test
    └── mediatype-test.cpp

Drafter could then depend on apib-parser.