Added SVG support (#366)

[sjroe]

Allows SVG to be rendered by:

• Using createElementNS for <svg> elements
• Replacing <blazor-component> with <g> when inserting components within an SVG tree

Addresses #366

[SteveSandersonMS]

Thanks @sjroe! This looks great. I'm impressed that you made the solution this simple. You can ignore the AppVeyor build failure as it looks like a random networking issue not related to your code.

Would you be able to add a simple test component for this in the test/testapps/BasicTestApp project (in a similar way to the other test components there), plus a test that uses it in the Microsoft.AspNetCore.Blazor.E2ETest project's ComponentRenderingTest.cs file? The test only needs to check that some SVG elements were inserted into the DOM correctly (as in, they can be located via Browser.FindElement(...)).

Once there's an end-to-end (E2E) test covering this, we'd definitely be happy to merge it!

[dnfclas]

All CLA requirements met.

[SteveSandersonMS]

Thanks again @sjroe - this looks totally perfect! It's now merged. Great to have this feature in :)