Fix #1506 Grid not aligned with ticks

[mistic100]

Add new grid.x.ticks options :

• integer value (default: 10) -> approximate number of lines to display
• 'auto' -> display lines aligned with axis ticks
• array -> manually define the lines

[codecov-io]

Current coverage is 63.97% (diff: 100%)

Merging #1855 into master will decrease coverage by 0.07%

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  Hits           2818       2818          
- Misses         1582       1587     +5   
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[panthony]

@mistic100 Would you be kind enough to provide a test with your fix?

That would be super awesome.

I know... It's 2018.

[mistic100]

I'm afraid I won't have time. I don't use this lib anymore.