Add MD5 support

[HossamMohsen7]

Hello,
To make sure the downloaded file is not corrupted you could use the "md5" tag in the json file from curse to check the md5 of the downloaded file

[timbru31]

Hi @7osCraft, this was the main reason I forked the project. You might want to switch to my fork, since this one is outdated/inactive.
You can find it here: https://github.com/timbru31/updater
(and ref for the commit which introduces MD5 sum checking: 8f7d4a9)