Improve how sending messages works

[amishshah]

Please describe the changes this PR makes and why it should be merged:
This is a WIP PR so I will fill this in later

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

Suggestions / Roadmap:

• Add an Attachment class to allow for:

// single files
channel.send(new Attachment(file, 'image.jpg'));
// multiple files
channel.send([
  new Attachment(file, 'image.jpg'),
  new Attachment(file2, 'image2.jpg'),
]);

---

• Add content as a valid property of MessageOptions

---

• Allow directly sending RichEmbed objects:

channel.send(new RichEmbed(...));

[Hackzzila]

Why not also do something like this?

// Sends a file
channel.send(buffer);
// ex
channel.send(fs.readFileSync('foobar'));

// Sends normal content
channel.send(buffer.toString());

// Sends a file
channel.send(readStream);
// ex
channel.send(fs.createReadStream('foobar'));

[devsnek]

content is a valid property of message options after #1490 gets merged

[iCrawl]

#1731