[Feature] Add a Tupfile for building the project with Tup

This is more for my own personal use since I prefer Tup to GNU Make
nowadays, thus the README does not describe Tup as a requirement or as
the primary way to build Luvent.

Tupfile.lua

@@ -0,0 +1,14 @@
+local sources = {"src/Luvent.lua"}
+local tests   = {"tests/Luvent.spec.lua"}
+
+tup.rule(sources, "^ Running Luacheck^ luacheck %f")
+
+tup.rule(sources,
+         "^ Creating TAGS for Emacs^ ctags-exuberant --languages=lua -e %f",
+         {"TAGS"})
+
+tup.rule(tests, [[^ Running Unit Tests^ busted --pattern=".spec.lua" %f]])
+
+tup.rule(sources,
+         [[^ Generating Documentation^ ldoc --dir="docs/" %f]],
+         {"docs/index.html", "docs/ldoc.css"})