If the given location is a file rather than directory, source it.
update readme to show that loc can be a file
remove btype arg from -antigen-load, since it's now unnecessary

The tests in bundle-syntaxes.t checked that -antigen-load was receiving
btype=plugin for every test, so it provided no useful coverage anyway
(for that aspect).  The parser tests for btype, so that functionality is
covered elsewhere.
Merge pull request #92 from willghatch/source_file

If the given location is a file rather than directory, source it.