Continue the process even if posting a message to Slack is failed

[Chikashi-Kato]

This PR is addressing the issue #40. After merging this PR, slacktee continues the process even if posting a message to Slack is failed.
The content of #40 is below:

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Currently, slacktee exits immediately with the error code if it fails to send a message to Slack or some other errors happen (e.g. webhook URL is not configured).
However, this was not ideal behavior as a tee-like-command because it blocks piped subsequent commands.

Here is the example:

cat test.txt | slacktee -n | mail foo@bar.com
If webhook URL is not configured, slacktee fails immediately and following mail command cannot read the output of cat command.

cat test.txt | tee inaccessible_file.txt | mail foo@bar.com
Even if tee cannot open inaccessible_file.txt, tee outputs an error message to STDERR and then continues to pass the input to mail command.

In order to minimize the chance to interfere the operation of the piped subsequent commands, slacktee should try to follow the error handling behavior of tee command.

[Chikashi-Kato]

Since this PR is open long time but no one review it, I'm merged this.