Make max_text_bytes_per_part configurable

[ayushdg]

Is your feature request related to a problem? Please describe.
#77 adds support for longer strings and as a part of those discussions it makes sense to expose some advanced config options to users as well: #77 (comment).

Describe the solution you'd like
Expose these hardcoded values as config options that can be tweaked if needed.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[sarahyurick]

Closing in favor of #331.