Adding support for overriding number of threads

[joaori]

Currently the number of CPU cores of the host running the converter is used to define the number of threads. This may not be ideal for scenarios when not all CPU cores are allocated to the application.

My current use case is running the converter in a docker container:

• no matter how many CPU cores I allocate to the container, the number of threads will be fixed and based on total number of cores available in the host.

This PR adds support for defining the number of threads through command line parameter -t [ --threads ] , so that we're able to tweak the default setting.