Remove dependency on instaloader script #1

Shelling out to a Python script is:

1. Unreliable: there is a complete lack of error handling, download progress information, etc.
2. Insecure: always a possibility of shell injection (be aware: currently I do not sanitize any of the Ids).
3. Inconsistent: shelling out to a Python script is just not right for a web service written in Go.

Before this can be hosted anywhere publicly these will need to be addressed.

Potential solutions:

1. Rewrite instaloader (or parts of it) in Go.
   • Taking a look at the source this looks like an extremely daunting task.
2. Use an alternative Go library or program for downloading insta content (one that still works in $CURRENT_YEAR).
   • No luck whatsoever finding anything like this so far.