Add option for custom scripts directory

[jhelgert]

This PR adds an option for providing a custom scripts directory. This gives the user a lot of freedom since they can use more sophisticated unpinning scripts and additionally, for instance, even setup a proxy using a Frida script.

Btw, some of the built-in (i.e., httptoolkit's) unpinning scripts may not work, as some hooks assume that the custom SSL certificate to accept is stored inside the CERT_PEM variable; see here. This is probably also the reason why unpinning doesn't work for some apps, as mentioned in #36. Thus, maybe it makes sense to add a default certificate in another PR?