Re wrote the plugin in order to have more functionality and be more efficient

The summary of what I did is below. I'm afraid having multiple pull requests at the same time is not possible. So I'll just do one commit per feature, and I'll give you a simple summary.

This is the summary of changes I promised, so that you can understand what I did without looking at the commits. Also, I'll keep updating this when I make new commits if there's anything worth mentioning, so that it's it's easier to not get confused. :wink: 

Something to take into account. Most of the features that I implemented are not possible without the re-write, so asking me to not push the re-write is impossible. :smiley: 

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- I re-wrote the whole thing basically, improving it's structure and making it more lightweight.
  - I improved it's structure in a way that makes more sense, only loading it when it's needed, the workflow of the plugin is easier and smarter.
  - Made it lightweight in the way that it now is only loaded when it is on, or called on. And when it is off by default, or it is called off, it will completely unload anything that the plugin created or was using, thus making it more lightweight when not in use.
- Added system to recognize file types easily and do special things with each file type.
  - Added recognition system for `[Command Line]` buffers, in order to only use permanent line numbers on them.
  - Added recognition system for `[Vundle] Installer` buffers, in order to only use permanent line numbers on them, since you may want to know how many plugins you're using.
- Added method to disable numbertoggle by default, until called on, which solves issue #5.
- Line number's width don't jump around when changing line numbers mode, thanks to @ClementJunca.
- Added a method to specify the mode in which numbertoggle should be on by default whenever turned off or when off by default. Solving the issue #5 even better.
- Added way to call numbertoggle, on/off, from one to the other, with no problems.
- Added invert mode, which does the same but opposite effects (insert mode has relative numbers, normal mode has permanent numbers, etc.).
- Improved practically non existent documentation.
  - Made the Vim docs for numbertoggle follow Vim's standards, making them info full with the Vim help format.
  - Improved numbertoggle's README file in order to show almost the same info as docs, but be prettier.
  - Improved the README file adding a Vundle installation section.
- Fixed a couple of bugs, mostly made by me and the one expressed in issue #6.

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That's the summary, so that you can take it all in, the commits don't matter. :wink: 

Note that the original pull request started on the 1b6fe6b commit, anything after that are patch commits. Patch commits don't matter, they're just bug fixes and stuff like that kind of patch commits. :wink: