Make test to describe `Converter#convert`
Fix description for class method

It's `Sawarineko.nya`, not `Sawarineko#nya`.
Add encoding support to Converter (#2)

Support UTF-8, UTF-16BE, UTF-16LE, EUC-JP, Shift_JIS, EUC-KR, CP949.
Add `-e/--encoding` option to CLI (Fix #2)