Fix bug in alternated background

[johnferchermeli]

Description

• Expand header and footer behavior to simulate a margin top
• Add guard closure in header and footer to turn background white

Related Issue
resolves #137

Checklist

check with "x", if applied to your change

• All methods associated with structs has func (s *struct) method() {} name style.
• Wrote unit tests for new/changed features.
• Updated docs/doc.go
• Updated pkg/pdf/example_test.go
• Updated README.md
• Updated all examples inside internal/examples
• New public methods/structs/interfaces has comments upside them explaining they responsibilities
• Executed go fmt github.com/johnfercher/maroto/... to format all files

[codecov-io]

Codecov Report

Merging #138 into master will decrease coverage by 0.33%.
The diff coverage is 90.63%.

@@           Coverage Diff            @@
##           master   #138      +/-   ##
========================================
- Coverage   95.32%    95%   -0.32%     
========================================
  Files           9      9              
  Lines         534    559      +25     
========================================
+ Hits          509    531      +22     
- Misses         21     22       +1     
- Partials        4      6       +2

Impacted Files	Coverage Δ
pkg/pdf/pdf.go	98.54% <90.63%> (-1.46%)	⬇️

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