Task enabled

[veewee]

Q	A
Branch	master
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Documented?	yes
Fixed tickets

This PR allows for dynamically enable a task. Usefull for conventions so that you can configurably run a task or not through parameters.

Examples:

# grumphp.yaml
parameters:
    run_security_advisories: true
grumphp:
    tasks:
        securitychecker_roave:
            jsonfile: ./composer.json
            lockfile: ./composer.lock
            run_always: true
            metadata:
                enabled: "%run_security_advisories%"

[drupol]

This is going to be very useful, thanks !