spelling support for various modules

[sthibaul]

Is your feature request related to a problem? Please describe.

spd-say -o espeak-ng -s hello

pronounces "hello" instead of spelling it.

Various other modules have the same issue, it seems only the festival, ibmtts and ivona have support for it.

Describe the solution you'd like
It should be spelt.

Additional context
Possibly adding spell-out ssml tags will be enough to get it to work with ssml-powered syntheses such as espeak-ng.
For others, if they don't have an interface for this such as e.g. festival has, we would want to support it by requesting speaking the letters one by one, also see #905