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.

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "lib/catch"]
 	path = lib/catch
 	url = https://github.com/catchorg/Catch2
+[submodule "lib/cxxopts"]
+	path = lib/cxxopts
+	url = https://github.com/jarro2783/cxxopts