* Use move semantics instead of copy for functions
In some cases, a few more copies could be prevented by changing function definitions to accept parameters by const-ref, rather than by value, but I didn't want to change public signatures.
* Fix two use-after-move errors
* Add `cache_control` parameter to `set_mount_point`
Specifies the Cache-Control header value to return when specified. For example:
```
svr.set_mount_point("/assets", "public/assets", "public, max-age=604800, immutable");
```
* Add default for cache_control
Default to "no-cache", which is implicitly what is happening today.
* Change set_mount_point to accept Headers
* Don't use C++17 destructuring
Andrew Gasparovic