Add option to specify line break character used in pagination

[myheroyuki]

In some cases when getting multiple pages of table data it's preferable to have the pages divided by breaks other than form feed. This allows users to separate pages by whatever they'd like.

[codecov[bot]]

Codecov Report

Merging #193 (d4867a2) into master (27017ef) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
+ Coverage   94.11%   94.14%   +0.03%     
==========================================
  Files           5        5              
  Lines        2295     2290       -5     
==========================================
- Hits         2160     2156       -4     
+ Misses        135      134       -1     

Flag	Coverage Δ
macos-latest	94.14% <100.00%> (+0.03%)	:arrow_up:
ubuntu-latest	94.14% <100.00%> (+0.03%)	:arrow_up:
windows-latest	94.01% <100.00%> (+0.03%)	:arrow_up:

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Impacted Files	Coverage Δ
src/prettytable/prettytable.py	90.39% <100.00%> (ø)
tests/test_prettytable.py	100.00% <100.00%> (+0.12%)	:arrow_up:

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[hugovk]

Thanks!