Improve version information

Add version number and string as macros and symbols. Version numbers can
be easily compared, for example in the C preprocessor:

    #include <cmark.h>

    #if CMARK_VERSION < 0x020200
    #error Requires libcmark 2.2.0 or higher
    #endif

Storing the version in a global variable allows to check the library
version at runtime. For example:

    if (CMARK_VERSION != cmark_version) {
        warn("Compiled against libcmark %s, but using %s",
             CMARK_VERSION_STRING, cmark_version_string);
    }

The version should be updated whenever the public API is changed.

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