Add aliases to versions

Aliases are stored as plaintext files in the $NVM_DIR/alias dir.
They may store either an explicit version (v0.3.6) or an implied version
("latest"). The latter is a "moving target", and thus possibly
dangerous, but can be useful, too.

.gitignore

@@ -1,3 +1,4 @@
 HEAD
 src
 v*
+alias