Slacking Versions Save

It has been quite some time ! Here an up to date release. Thank you all for your contributions!

• AppVeyor CI for Windows (Thanks @vlovo https://github.com/coin-au-carre/slacking/pull/5)
• Fix curl clean memory_leak (Thanks @Kawabaumga ! https://github.com/coin-au-carre/slacking/pull/10)
• Emoji support (Thanks @alar77 https://github.com/coin-au-carre/slacking/pull/8)
• Allow setting a proxy (Thanks @vlovo ! https://github.com/coin-au-carre/slacking/pull/3)
• Category WebHook (Thanks @vlovo ! https://github.com/coin-au-carre/slacking/pull/7)
• Remove deprecated Channels interface in favor of Conversations one
• Wrong channel throws an error rather
• Fix Attachment problem see #11
• Better CMake
• Update nhlomann::json to 3.9.1

• For convenience, allow escape characters in text message and URL encode string.
• Added the possibility to disable throw exception
• Upgrade nlohmann json to 2.1.1

• CPR has been removed. Simple curl sessions has been created. Slacking consists now of two header files.
• Added No throw exception options in Slacking.
• CPR functionnalities are consistent with old version. Upgrading should have no impact.

Header only library including CPR and Nlohmann Json.