bench against commonmark instead of showdown

[Feder1co5oave]

If we want to be competitive on performance nowadays, our competition is markdown-it and commonmark. Showdown is the slowest converter out there, and on my slow machine it takes a lot to bench.
I think it's pointless to compare against the notably slowest one, but we definitely should compare against commonmark. This PR does that.

See #963

[joshbruce]

Good call. Flagging for 0.6.0 for now.

[Feder1co5oave]

Also showdown brought in a lot of (dev)dependencies that we can now drop.
Currently my dependency tree on my branch looks like this:

$ npm list
[email protected] /home/federico/marked
├─┬ [email protected]
│ ├── [email protected]
│ ├── [email protected]
│ ├── [email protected]
│ └── [email protected]
├─┬ [email protected]
│ └─┬ [email protected]
│   ├── [email protected] deduped
│   └── [email protected]
├── [email protected]
├─┬ [email protected]
│ └─┬ [email protected]
│   └── [email protected]
├─┬ [email protected]
│ ├─┬ [email protected]
│ │ └── [email protected]
│ ├── [email protected] deduped
│ ├─┬ [email protected]
│ │ └── [email protected] deduped
│ ├── [email protected] deduped
│ └── [email protected]
└─┬ [email protected]
  ├── [email protected]
  └── [email protected]

[joshbruce]

@Feder1co5oave: Can we add the README update to this PR and then I will merge this and #1046 and publish 0.3.13 - which just leaves the lint thing over at #1020...no rush on that one.

[joshbruce]

Sorry! Wrong button.

[styfle]

@Feder1co5oave Would you like to move forward with this PR?

I think it makes sense and we could also use this as a opportunity to upgrade all the versions of devDependencies since these are pretty outdated.