Image: Add an option to crop on a single web element

[PaperBag42]

Added a new --selector option that takes a CSS selector and crops on the first element that matches it. Other cropping settings will work relatively to the selected element's coordinates.
Example:

# Saves the 'Google' logo from Google's site
wkhtmltoimage --selector '#hplogo' https://google.com google.jpg
# Now it's just 'oogle'
wkhtmltoimage --selector '#hplogo' --crop-x 70 https://google.com google.jpg

Unfortunately, I am still struggling with an error. After I had only added the new member to the ImageGlobal class, I started getting a segmentation fault at the end of execution. I failed to find the source, so I am hoping I could get some help from someone who had already experienced the issue.

Anyway, I hope you like this feature!

[PaperBag42]

Oh nice, the segfault disappeared after cleaning and rebuilding. I guess I sould have tried that earlier ¯_(ツ)_/¯

[ashkulz]

Thanks for the contribution! Can you rebase it on the latest master and add some docs in ImageCommandLineParser::outputExamples so that it's clearer that you can do negative crop arguments with this change when a selector is specified?

[PaperBag42]

I added some examples, I hope they're clear.