Implement Add Page

[johnferchermeli]

Description

• Implement Add Page

Related Issue
resolves #141

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 #147 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
+ Coverage   96.24%   96.29%   +0.06%     
==========================================
  Files           9        9              
  Lines         584      592       +8     
==========================================
+ Hits          562      570       +8     
  Misses         16       16              
  Partials        6        6              

Impacted Files	Coverage Δ
pkg/pdf/pdf.go	100.00% <100.00%> (ø)

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