maktaba should detect correct plugin name for github-USER-PLUGINNAME/ on rtp

Similar to #112, `maktaba#plugin#Get` and related functions like `maktaba#library#Require` will detect the wrong name for plugin dirs named like `github-USER-PLUGINNAME/`, which is a convention that VAM uses. For instance,

``` VimL
VAMActivate github:ConradIrwin/vim-bracketed-paste
```

adds a plugin dir `github-ConradIrwin-vim-bracketed-paste/` to vim's runtimepath, and maktaba uses the whole dir name "github-ConradIrwin-vim-bracketed-paste" as the name of the plugin, whereas bracketed-paste is the actual plugin name that should be used.

Most of these plugins don't have an addon-info.json, so adding more heuristics to `maktaba#plugin#CanonicalName` would be the only way to determine the right plugin name (otherwise the addon-info.json name would take precedence).