Added directive '/*!contexts:re2c ... */'.

Added directive '/*!contexts:re2c ... */'.

Directive is a new type of re2c block that consists of zero or more
of the following configurations (example values are the defaults):
    line = "long @@;";
    sep = "";

Renamed configuration: 're2c:ctxprefix' -> 're2c:contexts:prefix'

Added configuration: 're2c:contexts:expr = "@@"' that allows to
override the way context markers are accessed (but nottheir names).

Removed configuration: 're2c:define:YYDISTTYPE' (use the new
directive '/*!contexts:re2c ... */' instead).