Update screenshots in the doc

[nojhan]

Screenshots in the documentation should be updated to reflect improvement from v2.2.

In the "Why Liquid Prompt?" section: use normalized screenshots for at least:

• default theme
• unfold
• 256-colors presets
• unicode presets

Questions: should we show some extern themes there?

[Rycieos]

Questions: should we show some extern themes there?

I think themes in the liquidprompt org should have links to their documentation. Other themes should be on the wiki page, which can be linked in the docs.

[nojhan]

Thus add:

• Screenshot of Powerline.
• Link toward the Wiki page.

[Rycieos]

I'm closing this as it seems like all the goals were accomplished here.