README and Function Documentation

[howardbaek]

Overview

I originally planned on working on the README, function roxygen comments, and the coqui TTS vignettes, but this branch grew way too much with all the commits for the README and the function roxygen, so I will work on the coqui vignette on another branch.

What I did

1. README: Gave an overview of text2speech, how to check for authentication, list out different voices, and synthesize speech with the different services
2. Added a play_audio() function that lets you play audio in a browser.
3. Deleted tts_synthesis.R and moved tts() inside tts.R because that's where all the tts_*() functions live.
4. Deleted tts_**_auth.R and tts_coqui_installed.R and moved the authorization functions for the different services inside tts_auth.R.
5. Renamed tts_**_authenticated() to tts_**_check() to avoid confusion with tts_**_auth()
6. Used @rdname to pul related functions into one documentation page.

Overall, the package shows more structure and documentation within its functions

[cansavvy]

After walking through these code changes synchronously with @howardbaek , I think this looks good and ready to go.

The Files changed tab is a bit confusing to follow because the code is reorganized, but there are no actual deleting or adding bits of code.