Accept file extensions other than `.bib'.

- new function: ebib-locate-bibfile
- new user option: ebib-bibtex-extensions
  default value: `(".bib" ".bibtex")'
Rename ebib-preload-search-dirs to ebib-search-dirs.
ebib-ensure-extension: do not add extension when one is already present.
fix bug in ebib-get-db-from-filename.
Properly implement ebib-ensure-extension.
Update doc string of ebib-bibtex-extensions.