#63 Support for U+1AB7 and U+1AB8 (German dialectology)

[lucius]

I would like to request support for U+1AB7 (COMBINING OPEN MARK BELOW) and U+1AB8 (COMBINING DOUBLE OPEN MARK BELOW). These are used in German dialectology (Teuthonista phonetic script).

https://www.fileformat.info/info/unicode/char/1ab7/index.htm
https://www.fileformat.info/info/unicode/char/1ab8/index.htm

These were added in Unicode 7.0 in 2014. Support for other characters in Theutonista script would be appreciated as well.