Add image convert to webp

[mhz-tamb]

Ticket: #

Issue

Using images lighter than jpg

Solution

Allows to convert images to webp like tojpg

Impact

None

Usage Changes

Added a simple example in the documentation

Considerations

Testing

Tests are included, and run without problems

[coveralls]

Coverage increased (+0.007%) to 94.203% when pulling 1d2795f on mhz-tamb:webp into 758c6db on timber:master.

[mhz-tamb]

Hello! Do I need any changes?
In travis my tests break down. I think this is related to this issue travis-ci/travis-ci#8435
Will this be ok if I check the existence of the imagewebp function in the tests for skip it?

[pascalknecht]

Any updates on this PR?

[jarednova]

@mhz-tamb @pascalknecht — thanks for your patience! I just got back last week from a paternity leave and this PR is next on my plate

[jarednova]

@mhz-tamb @pascalknecht thanks again for the PR and your patience. As @mhz-tamb points out there is a Travis limitation on testing for webp — that said, things pass for me locally. I want to make sure we account for webp in the future, so here are two possible mitigations:

1. Using the guidance of #8435 figure out how to enable imagewebp in the PHP Travis build we're using
2. Skip the tests if imagewebp is not available.

@mhz-tamb — can you pick things back up from here?