Add Decode support for input_id sequences that don't start at a word prefix

[georg-jung]

Also, the Decode method currently assumes the first input_id you pass as its argument represents the beginning of a word. If the first passed id represents a word suffix - or, read: some arbitrary position in a list of input_ids, which doesn't happen to represent the start of a word -, it will probably throw a KeyNotFoundException. This probably isn't the best way how this method could work and I'm happy to fix that - if it helps you I could do it quite soon, please let me know.

#38 (comment)