Add simper spec runners (with make)

Allow usage of `make spec` to run project's specs against default
Crystal compiler or one that can be supplied with `CRYSTAL`
environment variable.

Also provide ways to run in automated mode thanks to watchexec.

[skip ci]

Makefile

@@ -0,0 +1,18 @@
+CRYSTAL ?= crystal
+
+profile ?= ## Display profiling information after specs execution
+verbose ?= ## Run specs in verbose mode
+
+SPEC_FLAGS := $(if $(profile),--profile )$(if $(verbose),--verbose )
+
+.PHONY: default autospec spec
+
+default: spec
+
+# `autospec` task uses `watchexec` external dependency:
+# https://github.com/mattgreen/watchexec
+autospec:
+	watchexec --exts cr --watch spec --watch src --clear $(CRYSTAL) spec $(SPEC_FLAGS)
+
+spec:
+	$(CRYSTAL) spec $(SPEC_FLAGS)