Version is defined in one place now.

There were redundancy: multimarkdown version is specified in parser.h and in
makefile. This change removed version from parser.h, so the version is defined
in one location now. When you want to bump version, only one file should be
edited.

Makefile tricks explained:

    override CFLAGS += -D MMD_VERSION='"$(VERSION)"'

1.  Without `override` asignment works only if CFLAGS is *not* defined in the
    make command line.
2.  Double quotes are required because code expects MMD_VERSION is string.
3.  Single quotes are required to protect double quotes -- otherwise shell will
    strip double quotes.