Add an option "--encoding"

This option is used to specify the encoding of the given TOC text file.
The default encoding is UTF-8.
Add Update operator
Delete incomplete download files

In some cases, there may be network errors while downloading updated
files, PdfMate will delete the incomplete files next time you invoke
update operator.

In addition, for specifying TOC file encoding, PdfMate will print out
available encodings when the input one is invalid.