Create option to set hyphen char in LuaTeX

[typiconman]

From my discussion with @pgmmpk:
Hyphenation char in LuaTeX can be set using:
\textchurchslavonic{\prehyphenchar=`-}%
in the body of the document.

By default the hyphenation character should be the hyphen, and if an underscore is desired, it should be set using an option, which invokes the above code in LuaTeX.
In XeTeX this is set by specifying HyphenChar when loading the font.

Alternatively, we should describe all of this in the documentation if we don't create the options.