Generate makefile with gyp

- Add gyp submodule
- Add script to generate makefile with gyp
- Switch travis build to do this

script/bootstrap.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+set -e
+
+git submodule update --init
+externals/gyp/gyp tree_sitter.gyp --depth .
+externals/gyp/gyp tree_sitter.gyp --depth . --format=make

script/ci.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+make compiler_specs
+out/Default/compiler_specs
+
+make runtime_specs
+out/Default/runtime_specs