Add simple markup language for changing the voice, voice pitch and voice rate

[drmfinlay]

I have received via e-mail a request to add to this application the ability to change the current text-to-speech language via markup in the input text. For example:

[lang="pl"] Text in Polish.
Text in Polish - next line.
[lang="en"] Text in English.
Text in English - next line.

The preferred voice for the language, if set in the application or system settings, can be used. An index might also be specified. For example:

[voice="pl-1"] Text in Polish voice one.
[voice="pl-2"] Text in Polish voice two.

Setting the voice pitch and rate might be done in the same way:

[pitch=0.75] Text spoken with a pitch of 0.75.
[rate=2.0] Text spoken with a rate of 2.0.

These options could be combined:

[lang="pl", pitch=0.75, rate=2.0]

The application settings should be restored at the end of audio playback or file synthesis.

I would say this is related to issues #17 and possibly #18, since markup text would not be included in the output.