Add a flag to make lighthouse portable across machines (#1423)

## Issue Addressed

Closes #1395

## Proposed Changes

* Add a feature to `lighthouse` and `lcli` called `portable` which enables the `portable` feature on our fork of BLST. This feature turns off the `-march=native` C compiler flag that produces binaries highly targeted to the host CPU's instruction set.
* Tweak the `Makefile` so that when the `PORTABLE` environment variable is set to `true`, it compiles with this feature.
* Temporarily enable `PORTABLE=true` in the Docker build so that the image on Docker Hub is portable. Eventually I think we should enable `PORTABLE=true` _only on Docker Hub_, so that users building locally can take advantage of the tasty compiler magic. This seems to be possible by setting a Docker Hub environment variable: https://docs.docker.com/docker-hub/builds/#environment-variables-for-builds

## Additional Info

Tested by compiling on a very new CPU (Intel Core i7-8550U) and copying the binary to a very old CPU (Intel Core i3 530). Before the portability fix, this produced the SIGILL crash described in #1395, and after the fix, it worked smoothly.

I'm in the process of testing the Docker build and running some benches to confirm that the performance penalty isn't too severe.

book/src/docker.md

@@ -29,6 +29,10 @@ $ docker run sigp/lighthouse lighthouse --help
 > Note: when you're running the Docker Hub image you're relying upon a
 > pre-built binary instead of building from source.
 
+> Note: due to the Docker Hub image being compiled to work on arbitrary machines, it isn't as highly
+> optimized as an image built from source. We're working to improve this, but for now if you want
+> the absolute best performance, please build the image yourself.
+
 ### Building the Docker Image
 
 To build the image from source, navigate to