Add support for specifying parent container

[jonjaques]

I've updated this PR to include a few tests and squash it down to 1 commit. Closes #14.

There was one hiccup I did not anticipate originally: changing the bar to be absolutely positioned within its container (by default the body), prevents it from being visible when the document is scrolled.

My solution was to set the position to fixed only when the parent is the body.

[cryptiklemur]

Throwing a big +1 to this one, would be super handy

[k-k]

When I first saw NProgress I said "Neat!", second thing I said was "wonder if I can change the containing element..."

+1 on this PR

[5im0n]

+1

[scarfacedeb]

+1.
I was thinking along the lines with this one.

[zwily]

👍

[joscha]

👍
@rstacruz can this be merged please?

[klyrr]

+1

[silvermanners]

+100 for this feature.

[brickgale]

👍 pls add this

[joscha]

@rstacruz any way we could get this in? Pretty please...

[chriserik]

+1

[rstacruz]

Looking at this now. I think I'd like to see this refined a little further to be merge-ready:

• This PR isn't mergeable anymore since the migration away from jQuery, so it'll have to be implemented again.
• If the parent container isn't specified (!parent), it should default to 'body', rather than the parent option being "body" by default. This allows us to...
• ...keep position: fixed when no parent is specified. If the absolute is the default, makes the progress bar not visible when the page is scrolled down.
• the #nprogress element should have a custom class if a parent is specified. This allows us to do something like #nprogress.np-custom-parent .bar { position: absolute; } without making position: absolute the default.

[jonjaques]

Updated PR here: #86

[rstacruz]

This has now been implemented. Thanks @jonjaques!

Documented here: https://github.com/rstacruz/nprogress/blob/master/Readme.md#parent