Feature request: Plain text console output

Please provide the following details if possible or relevant:

- [] the version of Ace impacted by the issue (as returned by the `ace -v` command).
1.0.0

Feature request: I'd like to add Ace checking to our build process at Standard Ebooks, to alert producers to accessibility issues in their productions before they submit them for review. Our build process runs on the command line, and can be instructed to run `epubcheck` after building to ensure the final epub is valid. It would be very helpful to be able to add `ace` to that step, but right now the console output of `ace` is verbose by default (a lot of info notices even when everything is OK) and the JSON output is tough for humans to read.

I'd like to suggest that `ace` be silent by default (the [rule of silence](http://www.linfo.org/rule_of_silence.html)), and instead of outputting JSON to the console, output a list of plain text errors/warnings a la `epubcheck`. Info notices and debug info could still be an option with `--verbose`, and JSON output could still be an option with a flag too. Then we could very easily integrate it into our build process, since the `ace` tool would then better follow the [rules of Unix](https://en.wikipedia.org/wiki/Unix_philosophy#Eric_Raymond.E2.80.99s_17_Unix_Rules).

(It may also be worth it to consider mirroring `epubcheck`'s output style as closely as possible, since conceivably both `ace` and `epubcheck` could be used simultaneously by ebook developers, and having the output in similar styles would go a long way towards ease-of-understanding.)

Thanks for all your hard work!