Support creating NuGet packages using both .csproj info and NuGetPackSettings info

### What You Are Seeing?

Using the `NuGetPack(FilePath filePath, NuGetPackSettings settings)` command where filePath is pointed at a `.csproj` file, you can create a NuGet package with the dependencies defined based on the .csproj file references. However, the NuGetPackSettings (i.e. the `settings` parameter) are only partially used. A .nuspec file is not created and the settings are only used to determine command line parameters. Settings like the `Files` property are ignored.

Using the `NuGetPack(NuGetPackSettings settings)` command, you can create a NuGet package that uses your `NuGetPackSettings` definition (from the `settings` parameter). This allows you to specify additional files to include (e.g. symbols). However, the .csproj references are not used and must be defined in the `NuGetPackSettings`.

We use Proget which allows a regular nuget package to contain both the assemblies and symbols (i.e. you do not need a separate nuget package for symbols).

Given the current behavior above, we can create a NuGet package that has dependencies automatically defined or a NuGet package that contains symbols, but not a package that has both.
### What is Expected?

The `NuGetPack` method should allow the user to specify if the `NuGetPackSettings` should be used to generate `.nuspec` file in the same folder as the project file (i.e. if the filePath points to a project file, a .nuspec file will be automatically created in the project file's folder). The `nuget pack` command will still be passed the .csproj file as its file parameter, but the NuGet tool will then detect that the .nuspec file exists and use both .csproj/.nuspec information (refer to https://docs.nuget.org/create/creating-and-publishing-a-package in the "From a project" section).

It appears that the Cake repo already has the majority of the required code. I would propose that a new setting is added to the `NuGetPackSettings` class (e.g. `bool CreateNuspecFileForProject`).
If true and the filePath is pointed to a .csproj, the `NuGetPackSettings` would be used to create/update the .nuspec file in the project's folder prior to execute the `Run` method (in the `NuGetPacker` class).
### What version of Cake are you using?

0.13.0
### Are you running on a 32 or 64 bit system?

64-bit
### What environment are you running on? Windows? Linux? Mac?

Linux, Windows
### Are you running on a CI Server? If so, which one?

Jenkins, Teamcity
### How Did You Get This To Happen? (Steps to Reproduce)

Start by inspecting NuGetPackSettings and the NuGetPacker Run flow, including the existing code related to .nuspec generation. Trace how a .csproj path and settings are currently handled, then verify the combined behavior for dependencies and additional Files. Done means the opt-in behavior creates or updates the .nuspec beside the project and the resulting package includes both sources of information.

Written by the indexing model from the issue text.