[Suggestion] Add php-pcov to php-*dev

[allansun]

First I'd like to thank to your work to bring this brilliant tool bundle, which saves us a lot of time to jump start our projects.

One thing I feel missing from the bundle is php-pcov. Even though we can generate coverage report by xdebug, its slowness and inaccuracy is unbearable.

In our projects we aim to have over 95% of code unit test covered, with xdebug it is unachievable, simply because the mechanism xdebug calculates code coverage is not accurate.

Whoever using the webdevops images I assume size of image is already not an issue, so I do hope we can include pcov in the bundle to make it even better.