Add possibility to store things in variables

[GuillaumeGomez]

Something like:

store-property: ("selector", "property", |variable_name|)
store-attribute: ("selector", "attribute", |variable_name|)
store-css: ("selector", "css", |variable_name|)
...

It'll require to check that variables |var| are correctly handled in all commands too. For example:

store-property: ("selector", "clientHeight", |clientHeight|)
assert-property: ("selector", {"scrollHeight": |clientHeight|})

Implementation details: when creating a new tests, it should clone all variables coming from the environment into a "local" dictionary so if they are modified, it won't impact other tests.

[GuillaumeGomez]

Can now be closed.