allow CSL directory to be user-configured

[bdarcus]

As more applications start to use CSL for style configuration*, there might be value in allowing a user-configurable style directory, and perhaps consider a default like '~/.csl/styles'.

• the new application Mendeley, for example, stores CSL files in its application package, and most/all of them are taken from the Zotero repository. I personally use CSL with both Zotero and Pandoc + citeproc-hs.