Simplify user configuration

[quentint]

Simplify user configuration by already pre-setting map and packages properties in System.config call argument.

User configuration is now easier, redundant calls like this are a thing of the past:

const map:any = {
map: {
'key': 'val'
}
};

Here's how to use it now:

const map:any = {
'key': 'val'
};

Same goes for packages.

ps: PRing this because it took me a while to figure out why my configuration wasn't working. This small improvement may help beginners!

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[quentint]

Just signed the CLA!

[googlebot]

CLAs look good, thanks!

[quentint]

Realizing I still have issues with SystemJS user configuration.

This PR may help, but it's still not enough for me, will dig into the docs a little more...

[quentint]

Forget it, this PR is dumb!

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