Don't treat due date and start date as special attributes.

Just consider them tags. Moreover, I'm dropping support for the 'start'
tag, which makes things a bit more simple. The 't' tag will serve as the
start date, it appears this is the standard among todo.txt users.
Rename setTag to setTagValue for sake of consistency.
Add 'dot' subcommand to 'dep' for making Dot graphs of dependencies.

The dot subcommand takes a task number or a text filter.

When given a task number, all direct and indirect dependencies of the
given task will be represented in the Dot graph.

When given a text, all tasks containing that text will be used and all
dependencies between those will be visible. This can be used to filter
for a certain project or context.

Example:

    todo.sh dep dot +SecretProject

The output still looks a bit crude, might be tuned later on.