Support mappings customization

As @achalddave mentioned in issue #73 there is no way to remap keys outside plugin, e.g. plugin contains only script-local functions `s:MergeBranches`, `s:JumpToHead`, etc. And when I disable Ctrl mappings via

```viml
let g:Gitv_DoNotMapCtrlKey = 1
```

It's impossible to map keys back

```
" This does not work
au FileType gitv nnore <buffer> <silent> <C-n> <SID>JumpToCommit(0)<cr>
```

To fix that:

* Define `<Plug>` mappings for used commands. So now it is easy to remap them

```viml
au FileType gitv nmap <buffer> <silent> <C-n> <Plug>(gitv-jump-forward)
```

* Use `autocmd` to map keys, e.g. no need to call `s:SetupMappings` on each
  buffer load.

* Update documentation to reflect the changes. Most of the mappings is used only
  for `normal` mode and keep mode column is too verbose.