Request: Add link validation feature

[Hebgbs]

Sometimes, especially if it seems more convenient to do so, some may wish to validate their URLs, and oftentimes for DownGit this means immediately preparing files for download, rather than adding in the additional step of validating if the link points to something which exists.

For convenience, and to save you bandwidth, it may be in your interest to add a check routine which can be triggered separate from the download routine, so people who are editing DownGit URLs by hand can check to see if their addresses are valid, rather than going through the process of preparing an archive such users would not intend to download.

[MinhasKamal]

Thanks for the feature request. We will look into it more and try to add this in the next release.