Alternate dictionary files

It would be useful if `helm-dictionary` could take an optional argument specifying the dictionary file. Then one could create different commands for different languages.

It would also be useful to allow `helm-dictionary-database` to be a list of files; the word list in this case would be the concatenation of all dictionary files.

Start by locating the definitions and callers of `helm-dictionary` and `helm-dictionary-database`. Trace how the current dictionary file is selected and how words are loaded, then verify that an optional file argument supports separate language commands and that a list of files produces one combined word list.

Written by the indexing model from the issue text.