Pass relative to absolute date conversion an optional offset.

If offset is not defined, it defaults to today (previous behavior).
Add new subcommand for postponing tasks.

It sets the due date to an absolute date or a relative pattern, for one
or more todos.

Example:

    todo.sh postpone 1,3,8 1w

This postpones tasks 1, 3 and 8 with one week, but only those that have
a due date set.
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.
Work with chomped source lines.

Should fix the issue that todo.sh does not echo a newline at the end of
the list of todos passed to the filters / sort commands.
Indent output properly.