v.0.3.1

Added UnixSeconds as output format, show builtin output formats in help

SHA 256 sums:

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315d572d8fa75424b75f7dc4035d86bdbfdf3634a135ba992ed80a4be5f484d9  timeconverter_v0.3.1_darwin_amd64.zip
c12b8382954bd20107e65d3a5f2676f6451f3a92a6a0db153b617a3dcbabe63e  timeconverter_v0.3.1_linux_386.zip
0259bb78e18d9f474f56874187d7c773a094207e85262f2b9cbde078ee933c68  timeconverter_v0.3.1_linux_amd64.zip
24cda1480ab8b61eadb87f61d2b588407fc76c578c439bae8f68da165610eab2  timeconverter_v0.3.1_linux_arm.zip
a9deb51c3f1a8647df6c21138f9f6640505006435c5519a0f63e61b42cd062fd  timeconverter_v0.3.1_windows_386.zip
e9585fee44ff179469a3ed25381dd9fad2da230a584f1ab3d45050dd11264b1d  timeconverter_v0.3.1_windows_amd64.zip