Feature: add a new git_prompt_explain command to describe the current prompt to new-comers

[sandrotosi]

Hello,
i just set up this in my bash terminal and it looks very nice! I'm sure at some point i'm going to remember all the symbols and stuff, but for now some of them are still a bit unclear what they are.

so it would be extremely helpful to have a new command, git_prompt_explain that provides a textual description of the terminal.

for example:

(extra info) [master ↑·7|✔]>

> git_prompt_explain
current branch: master
the branch is 7 commits ahead
the branch is clean

or

(extra info) [morph L|✚ 1 …2]>

current branch: morph
this is a local-only branch
1 file has been changed but not staged
there are 2 untracked objects in the repository