Add location_index and description field to output, add shared target to makefile

[krashish8]

Issue

Fixes #403, #617, #625

Tasks

• Add description key to unallocated tasks in the output, if provided (add 'description' to unassigned #403)
• Add location_index key to to unassigned tasks and each step, if provided (Adding location_index field in output where the location field is present #625)
• Add shared target to makefile (make shared), for creating Position Independent Code (Adding -fPIC flag in makefile? #617)
• Update docs/API.md
• Update CHANGELOG.md
• review

[krashish8]

It's funny how after adding the description and location_index keys, I realized that these changes only affect the end JSON-API users. The users that are using libvroom in C++ are not affected, and they could get this done even earlier. So, in truth, this change won't make any difference to us, but might be a bit helpful to the community! :-)

[jcoupey]

Thanks for the PR!

this change won't make any difference to us, but might be a bit helpful to the community!

You're definitely earning some open-source karma points here. ;-)