Node_exporter: Distinguish between enabling collectors additively vs. by enumeration

[cubranic]

The discussion in #735 made it clear that on one hand the dev team is concerned about surprising users upgrading to a new version, and on the other hand a subset of users (myself included) desires the pre-0.15 behaviour of exactly enumerating which collectors should be enabled.

It seems like both sides could be addressed by having --enable.{collector} act additively, and using a different option to define the exact collector set. (E.g., --collectors X,Y,Z.) Arguably, users opting for the latter option are experienced enough where they understand they have to read the upgrade notes for any collector name changes.

[SuperQ]

We usually try and avoid two ways of doing the same thing.

Also, what is the behavior / presidence of using both at the same time?

[SuperQ]

One thing to note, we're approaching 1.0 here, and with that we would have stability gauruntees that would make it less of an issue to include the no-defaults flag.

[cubranic]

The simplest way to handle using both is to exit with an error and a message that "--enable" and "--collectors" options are mutually exclusive and can't be used at the same time.

[pgier]

I would prefer to just have a flag which deactivates all the default collectors except for the ones that have been explicitly activated.

[discordianfish]

Agree with @pgier - maybe we should close this but fix #735?
Just for the record, I think a disable-all flag is and was fine to add. if @pgier agrees, I'll submit a PR.