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.

Now the following is easy to perform:
```
pyexcel.io.READERS['tsv'] = partial(pyexcel.io.csvbook.CSVBook, dialect='excel-tab')
merge_all_to_a_book(glob.glob('*.tsv'), 'out.xls')
```