Support parsing UTF-8 charmap file from locales

In addition to parsing the UnicodeData.txt, fall back to reading locales'
/usr/share/i18n/charmaps/UTF-8.gz file. Note that since is compressed, and
neither GZip nor Vim provide a very convenient way to reading the compressed
file, the implementation is clearly slower: the file is copied and uncompressed,
which implies some overhead.
Make documentation better and up to date