Implement columns iterator

[Jerome1337]

PR Details

Implement worksheet columns iterator

Description

Create an iterator function that will loop over every columns
Create a Next() and Error() functions like the rows iterator

Related Issue

Fixes #648

Motivation and Context

In some cases it is very helpful to iterate over columns instead rows
For example reading or writing vertical table

How Has This Been Tested

Tested using Go tests
Manually tested inside a personal project depending on Excelize

Every tests has been run on Ubuntu 18.04 and Go 1.13.4

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #653 into master will decrease coverage by 0.03%.
The diff coverage is 91.54%.

@@            Coverage Diff             @@
##           master     #653      +/-   ##
==========================================
- Coverage   96.45%   96.41%   -0.04%     
==========================================
  Files          30       30              
  Lines        8718     8789      +71     
==========================================
+ Hits         8409     8474      +65     
- Misses        189      192       +3     
- Partials      120      123       +3     

Impacted Files	Coverage Δ
col.go	98.25% <91.54%> (-1.75%)	⬇️

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[xuri]

LGTM, thanks @Jerome1337!