vimrc: set terminal encoding to UTF-8.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
vimrc: don't highlight Vim help as AsciiDoc.

Limit interpreting *.txt as AsciiDoc to files in $HOME.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>