ISSUE-162, add method to get total number of pages

[happsie]

Description
Adds a PageCount to maroto. Useful for adding conditions
Related Issue
Resolves #162

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 #167 (5fc41e3) into master (0d2e5af) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
+ Coverage   97.08%   97.09%   +0.01%     
==========================================
  Files           9        9              
  Lines         547      548       +1     
==========================================
+ Hits          531      532       +1     
  Misses         12       12              
  Partials        4        4              

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

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[johnfercher]

Hello, the GetCurrentPage method doesn't do the same thing?

[happsie]

@johnfercher sorry for late reply, totally missed this!
I though that would do too. But since this issue came up I thought that the requester had come up with a special case in his work when this would actually be needed. Might close this.