Handle NuGet Build .props .targets

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Unless I'm mistaken, Paket does not yet hook in .props and .targets files as NuGet has done since 2.5; I believe it should.

The primary reason is that more often than not, it presents an appropriate way for packages to manage adjustments to the buld process such as this

Key differences between such .props (imported at top of .*proj) / .targets (imported at bottom of .*proj) and the .ps1 are:

• no reliance on VS
• can deal with TargetPlatform etc. without silent problems and/or need to reinstall packages
• doesnt induce diffs when swithcing build output type

Note that .props is wired into VS with an editor for editing MSBuild Properties entries.

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(Assuming I'm right and it's not done), either this should be declared as a standalone 'not done yet' thing on the homepage and/or mentioned as the only workardound for .ps1 support that will be implemented.

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Related: Closed NuGet Codeplex: Add ScriptCS package installation hooks with interventions from ao @dsyme

[forki]

conversation is happening at #516