Adding a conntrack input plugin

[robinpercy]

• Collects conntrack stats from the configured directories and files.

[sparrc]

some revisions needed:

• rebase
• limit line length to 80 chars
• update README and CHANGELOG
• provide some documentation or link to documentation on what conntrack is in readme
• add a dummy windows build file that doesn't do anything (see ntpq plugin for example)

[robinpercy-xm]

Revisions have been made. I've restricted the build to linux, rather than excluding windows.

[sparrc]

I'll merge and fix it, but @robinpercy please make sure that in the future you don't change permissions on any existing files in the repo.

your PR includes changing etc/telegraf.conf to mode 755, which is executable and not suitable for a configuration file (did you accidentally do a chmod +x on that file?)