Use package.json information to set user-agent string

[nbusseneau]

Warning: requires #56 to be merged.

This PR automates user-agent string with information coming from package.json. Be aware that I have absolutely no idea how the following line behaves on environments that are not node.js:

const packageJson = require('../package.json');

It works completely fine on node.js because package.json can be reached from node_modules, but if this script is to be used in projects not using node_modules or not including package.json it will not work.

Do such projects even exist? Should this library cater to them? I'll let you decide because I have very minimal experience with the whole JS/NPM stack and basically don't know what is possible or not.

[coveralls]

Coverage increased (+0.08%) to 62.055% when pulling 1637f19 on Skymirrh:feature/better-user-agent into 5280d9a on tdurieux:master.

[tdurieux]

I always focused on pure nodejs because leboncoin does not allow cross-domain requests thus this library would not work in the browser.

[nbusseneau]

Closing since this code is now obsolete following latest changes. It may come back in a different flavour depending on the outcome of the discussion in #53.