Parser/generator: implement PCRE mode

[DmitrySoshnikov]

Currently we support only JavaScript regexes. However, regexp-tree can be language-agnostic, and support any other formats/modes/syntaxes of regular expressions. Thus, transform, and traverse modules can be fully reused for any syntax.

E.g., a named capturing group in PCRE, and Python syntax is:

(?P<foo>bar)

Similarly, we should support other syntax specifics.

[DmitrySoshnikov]

We can have plugins directory with own parser, generator, etc.

src/plugins
  /pcre
    /parser
    /generator

The PCREGenerator class can extend default generator with own node types.

For parser we can copy original BNF file, and extend it with functionality needed for PCRE.