[Feature] Support the --markdown flag

[dadav]

It would be awesome to add the --markdown option for simply printing the plain page file.

If this is added, this tldr client would work with https://github.com/denisidoro/navi/tree/master

[dadav]

Like the node.js client https://github.com/tldr-pages/tldr-node-client/blob/master/README.md#usage