Add new readme tags, php version etc

[andreasnrb]

To improve user experience when adding new plugins via the built in installer give developers the ability to add supports PHP version tags in their readme file. Possibly other tags could be useful as well.
With this also comes a check against the users server environment and they get notified if the plugin is compatible or not similar to how the WordPress version check works.
For example, in readme:
PHP:5.2.*-5.5.*
PHP:5.2.*

This will also then work when upgrading a plugin. The user should get a notice if the environment requirements in the plugin has changed.

[andreasnrb]

Of course having a dependency file together with plugins/themes that outline requirements for PHP modules etc could also be a way but that is more complex. That file could be a composer.json file or some other way to provide meta data about a plugin/theme.

[SergeyBiryukov]

#23880, #24271, #12260