modifier updates

[intoeetive]

Allow variables to use multiple modifiers; closes #136
Enabled additional variable modifiers to be registered by add-ons; closes #1203

Docs: ExpressionEngine/ExpressionEngine-User-Guide#546

• allow specifying parameters for modifier by prefixing parameter name with modifier name. E.g. limit:characters="1" would work the same as characters="1" when using limit modifier (but also have higher priority)
• allow modifiers to be chained, e.g. var_name:limit:spellout.
• allow add-ons to provide their modifiers. They would need to be registered in addon.setup.php file and then be called with __call in ModifiableTrait. The method_exists checks would then need to be updated to also check whether modifier is registered by add-on

[intoeetive]

@bryannielsen I made the necessary updates so that File field modifiers could be chained.

As of moving the __call method contents into separate function, I figured out that is not necessary - implementing __call in fieldtype level does not make any effect, because all methods require certain syntax to get parsed / processed - there's no chance someone would implement those