Allow importing apisprout from external packages

This can be useful for testing go code against the mock server.
The mock server could be started in a go test and the tests are not
dependant on a docker daemon or similar runtime.

To start a server or get a http.Handler without passing a file path or
URL, the server is separated into a server (`OpenAPIServer`) that only
requires an `openapi3.T` to run and the loading from a file or URL is
handled by a `ConfigReloader`. It can load from a URL or file path and
reload the `OpenAPIServer` on file changes.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

Author: leonnicolas

Dockerfile

@@ -3,7 +3,7 @@ WORKDIR /apisprout
 COPY . .
 RUN apk add --no-cache git && \
   go get github.com/ahmetb/govvv && \
-  govvv install
+  govvv install ./cmd/apisprout
 
 FROM alpine:3.8
 COPY --from=build /go/bin/apisprout /usr/local/bin/

README.md

@@ -61,7 +61,7 @@ Download the appropriate binary from the [releases](https://github.com/danielgta
 Alternatively, you can use `go get`:
 
 ```sh
-go get github.com/danielgtaylor/apisprout
+go get github.com/danielgtaylor/apisprout/cmd/apisprout
 ```
 
 ## Extra Features