Remove explicit dialect to allow keywords
Remove explicit dialect to allow keywords

The dialect specified is the default, so it is unnecessary to provide it. Allowing `keywords` to include `dialect` enables the easy construction and registration of additional (e.g. `dialect='excel-tab'`) readers and writers.
chfw/pyexcel,lordakshaya/pyexcel,chfw/pyexcel,lordakshaya/pyexcel,lordakshaya/pyexcel