Add JSDoc plugin

[pvdlg]

Related xojs/eslint-config-xo#45

Fix #282

I added the plugin and rules suggested in #282. I didn't add the --jsdoc option for the reasons explained in this comment:

As the valid-jsdoc rule is used when a JSDoc bloc is defined, in the end the jsdoc flag would just enable the require-jsdoc rule forcing to write JSDoc for every declaration in the code.

Enforcing 100% JSDoc is not really something very common from what I see in the open source community (AFAIK ESLint is the only major project with near 100% JSDoc coverage).

In addition, it would be difficult to find the best default config for the rule. Some might want a warning, some an error. Some might want a JSDoc block for function expression and declaration, some for declaration only etc...

So I'm not sure adding a flag is justified for a case that is not really common, and requires additional config anyway. It's probably better to just set the rule in XO:

{
  "xo": {
    "rules": {"require-jsdoc": ["warn", {...}]}
  }
}

@sindresorhus if you think the --jsdoc flag makes sense I can add it. Not sure what the require-jsdoc options would be though.

[sindresorhus]

if you think the --jsdoc flag makes sense I can add it. Not sure what the require-jsdoc options would be though.

No, I agree with you. We should not add it.

[pvdlg]

Just rebased. As we won't add the --jsdoc flag it should be good to go