feat(template): use projectSlug as default for appName

[eljoth]

Working with the template, I encountered the following:

linting the repository that just was created, the linter errors due to not handling the error from the logger.Sync() call.

Error return value of `logger.Sync` is not checked (errcheck)
        defer logger.Sync()
                         ^

In addition I think it is more convenient to start with a cmd/{{.projectName}} instead of cmd/awesomecli if nothing else is specified.

[linuxluigi]

Thanks for you contribution, looks good for me 👍

[brumhard]

@eljoth I rebased your change to use the projectSlug as default for appName onto the new way of defining options. Hope you don't mind, thanks for the contribution 👍🏻
Fixes #54.