Allow for script to be selected from dropdown in Visualize

[rashidkpc]

The user should be able to input a script to be used for calculating the values of an aggregation. Because of the potential for class casting exceptions when a script is specified along with a field, we should not be sending a field when a script is in use.

[w33ble]

There's a lot of stuff that needs to happen given the current plan of attack. It's highly related to #1537 and is more about how the interface for scripts will work, and not so much about their implementation.

Currently, we'll planning to add a _script option to the field selection, and show a script input in the agg builder if selected. This seems pretty easy on the surface, but the change is pretty far reaching, since the vislib, the agg builder and basically everything else all expects the aggConfig.params to contain a field, which script queries will not have.

Most parts use the field param for displaying (ie. labels) - this may be solved by saved scripted fields, assuming we're going to require a name for every script that's used.

[w33ble]

Closing as part of #1537