added sendFile

[kylegoetz]

I wanted to be able to send files with hyper-ts but there was no way to use it natively. I had to write Express code and then use fromRequestHandler to turn it into a hyper-ts middleware.

One can use sendFile it in lieu of send. It will also transition the index from BodyOpen to ResponseEnded. It takes an error handler just like json does, and it will trigger the error handler if one of three things is true:

1. the path is a relative path (Express, for example, only allows absolute paths, so this is somewhat opinionated); I've also named the parameter in sendFile absolutePath to hint to an end user that the path must be absolute.
2. the path is to a file, not a directory
3. the file exists

I updated the ExpressConnection to work with this as well and added some unit tests.

[kylegoetz]

Also I started to add a @since 0.7 but wasn't sure if I should. I figure if the PR is accepted, a maintainer would correct that. I did not commit the updated documentation for the same reason.

[kylegoetz]

One thing to address: should sendFile attempt to auto-detect mime type and add the appropriate Content-Type header? I have not done this because the most straightforward way to do this would be to add another dependency that does this (there are so many mime types!). I didn't want to force a dependency.

I suppose sendFile could take an optional Content-Type parameter. Might that be the right way to do it? Or should we just expect the user to explicitly chain addHeader('Content-Type', 'application/pdf') for example? In my own project, this is what I'm doing.