Add log/gist-log commands

It will be very useful to have command that just opens chocolatey.log
It may be very boring to rerun installer with -v/-vd or to copy-past log file location from shell window, as it is very hard to not select surrounding brackets.

Another useful command may be gist-log inspired by [brew gist-logs "formula name"](https://github.com/Homebrew/homebrew/blob/master/share/doc/homebrew/Troubleshooting.md), e.g. upload log of package install to gist.