Add subdomain routing

[AlecDivito]

Adds support for routing on subdomains.

Description

Look at issue #690.

Motivation and Context

I needed support for routing on subdomains inside of my project.

How Has This Been Tested?

New tests have been added into the /tests/namespace.rs file.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• Added example of new feature

[Fishrock123]

@jbr Had a direction idea on this that wasn't posted to the original issue which I personally think is much better and more powerful than .subdomain().

It's possible that idea may not work out or may not materialize, but I think we should either consider waiting or consider that this api may later be dropped.

[AlecDivito]

Yes, he actually posted his idea inside of issue #690 but then soon deleted it. Though i still have his response. He proposed doing routing using the entire url like so:

app.at("/path/still/works").get(…);
app.at("http://*/only-http").get(…);
app.at("//example.com/any-scheme/but-only-example.com").get(…);

I agree that this would be a lot more powerful but I also think that it would also be a lot more work. I just assumed that because he deleted his comment it wasn't being considered anymore.

[joshtriplett]

I would love to have support for this, as well.