support for dynamic value of the textarea's rows attribute

[max-l]

msd-elastic allows setting the text area's row attribute with a constant.

ex: this works:

<textarea msd-elastic ng-model="field.value" rows="5"></textarea>

but not this :

 scope.rows = 5 

<textarea msd-elastic ng-model="field.value" rows="{{ rows }}"></textarea>

in the later case, the rows attribute is ignored