Fix bug in handling of `--xxx=yyy` format commands

[delphidabbler]

Commands in format --xxx=yyy (e.g. --datetype=modified) are detected only by checking that a command starts with a specific string, i.e. --xxx=yyy is detected by checking that the command string starts with --xxx. The position of the equals sign is detected separately.

This means that --xxx=yyy and --xxxabc=yyy are both assumed to be the same command.

[delphidabbler]

The whole TParams class could do with refactoring - when it only had to parse a few parameters it was fine, but now there are many more commands it's got messy with a lot of duplicated code.