Provide open custom reporter

[ghostbuster91]

Hi, I would like to ship the output from this plugin to the Firebase to have all data from my team in one place.
I saw that bryanstern started working on restReporter (https://github.com/bryanstern/build-time-tracker-plugin/tree/rest_reporter) which could be useful in that case.
On the other hand I saw that there are needs for other types of reporters.
I think that it might be a good idea to give user some freedom and allow them to easily create their own reporters in configuration. Of course we can still have some predefined base reporters like summary, cvs or the restReporter.

Such approach will allow people to customize their reporters as they need and if some reporter would be considered as commonly used it could be pulled into build-time-tracker-plugin.

Let me know that do you think about it.

Read below only if you agree
I don't have much experience in creating gradle plugins so if you agree to my proposition it would be very helpfully if you could give me some hints about possible way to implement.
Right now is see two following options (I think they are very simillar):

1. Create CustomReporter class which would accept closure with user's code to execute.
2. Allow users to implement some interface and provide such implementation through configuration.

[passy]

Love the idea!

I don't have much experience in creating gradle plugins [...]

Neither had I before I started this. :)

Your outline sounds good to me. I'm not sure if there's any form of prior art for customizations like this. I would not be surprised if there's a gradle plugin out there that allows configuration in that style that we could take some inspiration from.