Install dependencies from remote git URLs

[milancurcic]

As discussed in #33, let's allow installing fpm-enabled packages form their git repositories.

If the following is placed in fpm.toml:

[dependencies]
dependency_name = { git = git_url, tag = git_tag }

Then the package depends on dependency_name, which fpm will fetch from git_url and a specific git tag git_tag.

Here's a concrete example that will work, when this issue is implemented.

[dependencies]
datetime = { git = "https://github.com/wavebitscientific/datetime-fortran", tag = "v1.7.0" }

Question: If tag is omitted, should fpm fetch from the latest commit on master? I think so. Cargo does it like that as well. Then you could also do:

[dependencies]
datetime = { git = "https://github.com/wavebitscientific/datetime-fortran" }

[everythingfunctional]

This is exactly my plan.

[certik]

Yes, if tag is omitted, it should check out the latest master.

Otherwise I agree.

[everythingfunctional]

This was finished in #85