allows user to change the bottom margin

[boulama]

this allows the users to change the margin bottom of the pagination & "DONE" buttons.
they can use the "bottomMargin" argument to do change the bottom margin.

[codecov]

Codecov Report

Merging #27 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
+ Coverage   92.53%   92.61%   +0.08%     
==========================================
  Files           6        6              
  Lines         268      271       +3     
==========================================
+ Hits          248      251       +3     
  Misses         20       20              

Impacted Files	Coverage Δ
lib/src/fancy_on_boarding.dart	93.20% <100.00%> (+0.20%)	⬆️
lib/src/fancy_page.dart
lib/src/pages.dart	100.00% <0.00%> (ø)

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[xsahil03x]

@boulama Please resolve conflicts.