Add TODOTXT_DATE_FORMAT environment variable

[karkhaz]

TODOTXT_DATE_FORMAT sets the format of creation and completion dates. Prior to this commit, the format was hard-coded to %Y-%m-%d.

This commit fixes #456.

Reviewers: test this feature by setting TODOTXT_DATE_FORMAT.

export TODOTXT_DATE_FORMAT="%Y-%m-%dT%H:%M:%S%z"
t add Solve the halting problem
t list
# 1 2009-02-13T04:40:00+0000 Solve the halting problem
t done 1
t listall
# 0 x 2009-02-13T04:40:00+0000 2009-02-13T04:40:00+0000 Solve the halting problem

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There are a couple of quote warnings, not related to my code. Happy to fix in a different commit if needed.

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