Implement tab autocomplete

[reidenong]

Implements the tab autocomplete feature for both commands, directories and files.

Behaviour:

• Check if we are doing autocomplete for a command or a file

• If command, we do prefix matching among all commands and return a unique match

• If file, we find the possible file/directories based on given address and current position, then do prefix matching for unique matches

• In all cases, where there is no match, tab does nothing.

• In all cases, where there are multiple valid matches, tab does nothing (no guess).