Merge pull request #27 from raphael-proust/master

Vim bindings (#24)
Added the vim script to the Makefile
Added vim infos to README
README: fixed markdown syntax
Enable tab indentation

The configuration takes an additional `atom` field which indicates the string
used for indentation. Usual values include `"  "` (two-spaces) and `"\t"`
(tab) but other are possible. The other fields (`base`, etc.) indicate the
number of atoms to use for each kind of node.
Use character as identation atom

This simplifies the code (with an acceptable loss of generality).