Drop Boost dependency

Boost was only used for program option parsing. To avoid this huge
dependency, this commit replaces boost::program_options with cxxopts,
a header-only library with the same functionality.

cxxopts is added as a submodule, and Boost is removed from the
dependencies in the code and Travis configuration.

.ci/Dockerfile-ubuntu-18.04

@@ -3,7 +3,7 @@ FROM ubuntu:18.04
 ARG toolchain
 
 RUN apt-get update
-RUN apt-get install -y libboost-all-dev cmake git ninja-build re2c
+RUN apt-get install -y cmake git ninja-build re2c
 RUN if [ "${toolchain}" = "gcc" ]; then apt-get install -y g++; fi
 RUN if [ "${toolchain}" = "clang" ]; then apt-get install -y clang; fi