Add SimpleCommandParser class
Rename token.hh to Token.hh

This is a preparation for the another upcoming refactoring that will
merge Token.hh with SimpleCommandWord.hh.
Rename SimpleCommandWord to Token

The name SimpleCommandWord is too long. In addition, it is going to be
used to parse commands other than a simple command.
Rename CommandParserBase

The CommandParserBase class is going to be replaced by a new
implementation. The old implementation is temporarily kept in a
different name.
Redefine reset behavior of SimpleCommandParser

The token parser should always be reset into the same state, so we
should specify accepted token types.
Add new CommandParserBase abstract class
Rename CommandParser

The CommandParser class is going to be replaced by a new implementation.
The old implementation is temporarily kept in a different name.
Add new CommandParser class