Add a pause command?

[sthibaul]

Is your feature request related to a problem? Please describe.
Some users may want to have a pause e.g. between paragraphs.

This can be implemented several ways:

• using ssml <break> and get it implemented for all modules, see add some level of ssml support to module_strip_ssml #905
• adding a pause command in the spd protocol.

At least, for the former we will want to introduce a pause message type anyway, so it would probably make sense to make it available through the spd protocol too. Of course it would only be useful at priorities that do not interrupt themselves.