feat: batch processing API

[kxxt]

Add two sub commands that process a batch of inputs.

• batch-text
• batch-ssml

The input file to this two sub commands is of the following format:

/home/user/1.txt
/home/user/2.txt

It produces the following files:

/home/user/1.mp3
/home/user/2.mp3

As an improvement, we might add a --output argument that specifies how the output file should be named.

e.g. for ./1.txt, --output=/home/user/Downloads/[name].mp3 produces /home/user/Downloads/1.mp3 and --output=/home/user/Downloads/xxx[name].mp3 produces /home/user/Downloads/xxx1.mp3