Add command-line configuration override

Adds ability to override a setting provided in the configuration
file by passing a setting in a key-value pair format as command
line argument.
Ignore pickle files
Add failure messages in parallel execution

It makes Icarus report failures in the execution of an experiment
even when running in a process pool. Before, any issue would not
be shown to the user, who could know of a failure only by looking
at the result set.

In addition to that, it also prints in summary reports the
number of failed experiments.