Create command to show available `Forecaster`s

[victorgarcia98]

Following on the creation of the command flexmeasures show reporters (PR #686), this could be replicated for the Forecaster class. This command should look something like this:

Command Syntax

$ flexmeasures show forecasters

Example Output

Forecasters:

name              version  author     module
--------------  ---------  --------  --------------------------------------------------
Naive             1                   flexmeasures.data.models.forecasting.naive

This command should list all the forecasters available from the flexmeasures and from all the plugins registered to the instance, as well.