Optionally disregard shell env

See merge request twittner/tinylog!7

`True` by default, setting it to `False` will make the logger ignore environment variables.

See MR #5 for details.

This reverts commit 9bffbfe3.

Custom format

See merge request twittner/tinylog!3

This reverts commit 381a474c.

Co-authored-by: Matthias Fischmann <mf@zerobuzz.net>

See issue #1.

Misprint fixed

See merge request !1

Commit 5cf41025 introduced a bug which
prevented client code from being able to clear the date format in
settings. By implicitly wrapping the argument in a `Just` the actual
timestamp rendering kicked in which produced an empty field in the log
output.

This commit fixes this behaviour by changing `setFormat` to take a
`Maybe DateFormat` parameter. Client code can pass in `Nothing` to
disable timestamps.

As discussed in #3, this exposes the `DateFormat` constructor to allow
clients complete control over turning `UnixTime` values into
`ByteStrings`.

Needs GHC >= 7.6 due to System.Environment.lookupEnv