Allow to opt-out of targets import

[forki]

@ctaggart suggested we add a way to opt-out of targets import like in #574

in #604 (comment) he proposed the syntax for `paket.dependencies:

nuget Foobar 1.2.3 framework: net20 net35, autoimport: false

I don't really like the name autoimport. How about import_targets: false

/cc @agross

[agross]

import_targets is better. autoimport sounds like some magical thing will (not) happen. I don't like magic in code.

[ctaggart]

That is fine. Just note that it is .props too, just not .targets. The original feature called: Automatic import of msbuild targets and props files. http://docs.nuget.org/Release-Notes/NuGet-2.5

[forki]

I started by adding this switch to the paket.references file. see also http://fsprojects.github.io/Paket/references-files.html#import_targets-settings

[forki]

bringing this into paket.dependencies is now part of #587