Added support for titles in framed views

[conejoninja]

This PR add a new attribute "Title" to the views, if it's set to a non-empty string and the view has Frame = true, then the title is shown on the top border (replacing the long dashes).

IT has a 2 spaces margin, and titles are shortened automatically to fit the space available.

Screenshot:

Usage:

    if v, err := g.SetView("example", 2, 2, 30, 30); err != nil {
        if err != gocui.ErrUnknownView {
            return err
        }
        v.Frame = true
        v.Title = "This is a title"
        fmt.Fprintln(v, "Some content here ...")
    }

[jroimartin]

First of all, thanks for the contribution! :) Now, take into account that I plan to release v0.2 next Friday if no bug shows up, so, in case of merging the PR it would be for v0.3.

On the other hand, some time ago we discussed exactly about this very same feature (Issue #30) and my first thought was that we should keep views as simple as possible and handle title as views without frame, etc. However, I must admit that after seeing the screenshot, I'm seriously thinking about adding the feature :p Let me review the PR and we can consider merging it for v0.3.

[conejoninja]

I totally understood. This was a quick hack I did on the train. Didn't test it much.

I first tried the two views approach, but it gets messy fast if you have a few views (because position/size), plus you need to act on two views instead of one when updating/deleting.

But titles could be more complicated (align, colors, margins, ...), so I understand you don't want to include them in gocui.

[dcu]

nice!

[jroimartin]

I've just added the feature. For more details, please, follow the discussion on issue #30.