Add SSML or basic emphasis support

[drewster99]

Kokoro-iOS needs basic emphasis support

For example, all of these produce exactly the same result:

• You, without help, got the best outcome
• You, without help, got the BEST outcome
• You, without help, got the best outcome
• You, without help, got the BEST outcome
• You, without help, got the best outcome

Related hexgrad/kokoro repo

Replied Issue

In the hexgrad/kokoro repo, there's a related open issue for adding SSML support. In that repo, someone replied, saying you can slightly adjust things, like this:

💡 Customize pronunciation with Markdown link syntax and /slashes/ like `[Kokoro](/kˈOkəɹO/)`
--
 
💬 To adjust intonation, try punctuation `;:,.!?—…"()“”` or stress `ˈ` and `ˌ`
 
⬇️ Lower stress `[1 level](-1)` or `[2 levels](-2)`
 
⬆️ Raise stress 1 level `[or](+2)` 2 levels (only works on less stressed, usually short words)

Using kokoro>=0.9.2 from Python, I tried this, and I can get a slight emphasis, like this:

You, without help, got the [best](+2) outcome. -- emphasizes "best".
However, this was not the case using kokoro-ios.

Related PR

hexgrad/kokoro#302

Workarounds?

Are there any workarounds or any way to add emphasis to words or phrases today?
If not, how tough would it be to implement any of the above?

Thanks!