Add `get_formatted_string()` convenience function

[rickwporter]

In a couple uses of PrettyTable, I have added a similar function to allow for less code from my CLI projects. In those projects, I allow specifying the output format type as an arguments.

[codecov[bot]]

Codecov Report

Merging #241 (638a2ba) into main (03510e1) will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #241      +/-   ##
==========================================
+ Coverage   94.75%   94.83%   +0.08%     
==========================================
  Files           5        5              
  Lines        2326     2364      +38     
==========================================
+ Hits         2204     2242      +38     
  Misses        122      122              

Flag	Coverage Δ
macos-latest	94.79% <100.00%> (+0.08%)	:arrow_up:
ubuntu-latest	94.79% <100.00%> (+0.08%)	:arrow_up:
windows-latest	94.75% <100.00%> (+0.08%)	:arrow_up:

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Impacted Files	Coverage Δ
src/prettytable/prettytable.py	91.32% <100.00%> (+0.07%)	:arrow_up:
tests/test_prettytable.py	100.00% <100.00%> (ø)

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[hugovk]

Please could you document get_formatted_string somewhere in the README?

[rickwporter]

Please could you document get_formatted_string somewhere in the README?

Good call. I'm open to suggestions on the README. I was not quite sure where it best fit, or if my description length and content were appropriate.