add filter on extensions

[agallou]

It's now possible to add a filter on an extension.

atoum as a mechanism to ignore some tests if a php extension is not loaded. For example :

/**
 * @extensions blackfire
 */
class Example extends atoum

This change adds an extension variable to the filter rule.

You can now, for example launch only the tests that needs the blackfire extension :

./vendor/bin/atoum -d tests --filter '"blackfire" in extensions'

[jubianchi]

@agallou could you add a changelog entry please ?

[Hywan]

:+1:

[agallou]

changelog updated.

[agallou]

branch rebased after the merge of the PR #24