Feature Request: ExecShell

Throwing this up for discussion based on the conversations today around extension inference in exec.

Let's flesh out the requirements, and features this task should implement.
## Extension Inference

From the migration perspective, extension inference is a must, and which extensions are inferred should be configurable via a semicolon delimited list passed as a task parameter (`InferredExtensions`?)

Determining the first token in the passed in `Command` for this task, is a difficult but solved problem. We can take advantage of existing implementations of these algorithms.

For breaking apart the input string array into tokens we can use the implementation in CoreFX that does this:
https://github.com/dotnet/corefx/blob/master/src/System.Diagnostics.Process/src/System/Diagnostics/Process.Unix.cs#L437

After mutating the tokens, to recombine them into a single string for use by Process.Start, we can use the analog implementation of the above algorithm present in the cli:
https://github.com/dotnet/cli/blob/rel/1.0.0/src/Microsoft.DotNet.Cli.Utils/ArgumentEscaper.cs#L23

@rainersigwald @livarcocc @eerhardt @cdmihai @AndyGerlicher @Sarabeth-Jaffe-Microsoft