Ignore node_modules by default

[MoOx]

If I want to use prettier-markdown "packages/**/*.md" I just can't since it's the cli is not ignoring node_modules by default (in practise I should probably be able to, but a module I use have a malformed js snippet in its readme).

[DSchau]

Good call! I will be relatively busy until later this week, but I'll try and get that built in.

PRs also welcome and encouraged 😄

[DSchau]

But also meant to mention, thanks for using the tool and thanks for the feedback! Glad you seem to be getting some usage out of it :)

[DSchau]

@MoOx fixed; I will release this in a bit!

[MoOx]

Thanks!