Add some tooling to bring in Antlr4 for eventually parsing .otto files

The thoughts are still swirling around in my head a bit, but given the current
targets that Antlr4 supports, I fear that .otto file parsing will need to be
handled in JavaScript or C++.

Makefile

@@ -0,0 +1,42 @@
+
+
+ANTLR_BIN=antlr-4.7.2-complete.jar
+ANTLR=contrib/$(ANTLR_BIN)
+GRAMMAR=Otto.g4
+################################################################################
+## Phony targets
+all: help
+
+check: ## Run validation tests
+
+depends: prereqs $(ANTLR) ## Download all dependencies
+
+prereqs: scripts/prereqs.sh ## Check that this system has the necessary tools to build otto
+	@sh scripts/prereqs.sh
+
+clean: ## Clean all temporary/working files
+	rm -f $(ANTLR)
+
+
+parser: depends $(GRAMMAR) ## Generate the parser code
+	@for target in JavaScript Go Cpp; do \
+		java -cp $(ANTLR) org.antlr.v4.Tool \
+			-Dlanguage=$$target \
+			-o build/$$target \
+			$(GRAMMAR); \
+		echo "--> Generated $$target stubs"; \
+	done;
+
+################################################################################
+## Non-phony targets
+$(ANTLR): ## Download the latest ANTLR4 binary
+	(cd contrib && wget https://www.antlr.org/download/$(ANTLR_BIN))
+
+################################################################################
+
+# Cute hack thanks to:
+# https://marmelab.com/blog/2016/02/29/auto-documented-makefile.html
+help: ## Display this help text
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'
+
+.PHONY: all check clean depends

Otto.g4

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+grammar Otto;
+r  : 'hello' ID ;         // match keyword hello followed by an identifier
+ID : [a-z]+ ;             // match lower-case identifiers
+WS : [ \t\r\n]+ -> skip ; // skip spaces, tabs, newlines

build/.gitignore

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+*

contrib/.gitignore

@@ -0,0 +1 @@
+*.jar

scripts/prereqs.sh

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+#!/bin/sh
+
+err() {
+    printf '\e[48;5;%dm' 196
+    echo -n ">> ${1}"
+    printf '\e[0m \n'
+}
+ok() {
+    printf '\e[48;5;%dm' 034
+    echo -n ">> ${1}"
+    printf '\e[0m \n'
+}
+
+which wget &> /dev/null
+if [ $? -ne 0 ]; then
+    err "Please ensure that \`wget\` is in your PATH"
+    exit 1;
+else
+    ok "wget is present"
+fi;
+
+which java &> /dev/null
+if [ $? -ne 0 ]; then
+    err "Please ensure that \`java\` is in your PATH"
+    exit 1;
+else
+    ok "Java is present"
+fi;
+