[charmod-norm] Make coeng homonyms look better

r12a has just created a new issue for 
https://github.com/w3c/charmod-norm:

== Make coeng homonyms look better ==
2.2.4 Limitations of Normalization
http://w3c.github.io/charmod-norm/#normalizationLimitations

> the character sequences U+17D2 U+178F (្ស) and U+17D2 U+178A (្ដ) in
 Khmer

Showing these sequences as characters in the parens is not very 
helpful since (a) they lack something to combine with and get tangled 
with the parens, and (b) the similarity is not necessarily clear in 
such a context using default fonts.  I suggest using a hyphen or a 
Khmer consonant as the first item in the sequence, eg. 

-្ត -្ដ

or 

ក្ត ក្ដ



Please view or discuss this issue at 
https://github.com/w3c/charmod-norm/issues/111 using your GitHub 
account