Add User Defined Application Directory Flag

[Figuera]

This commit mimics the behaviour of nwggrid in which the "-d" flag ("--dir" in our case) override the directory in which we search for applications.

Ex: nwg-drawer --dir $XDG_DATA_HOME/application/games

Will display only the applications found in '$XDG_DATA_HOME/application/games' and nothing else.

This makes reference to issue #34 in which I was hoping to achieve the same behavior but using a different strategy. At the time you proposed a solution which worked fine but ended up not meeting my needs due to other problems. It seems however the branch with the solution was never merged and is now dropped. I believe this to be an overall better way to deal with the problem because has a more direct and intuitive approach providing much power to the user select what application he wants to show up.

I must do a disclaimer that I am not in any way an experience Go programmer, in fact I mostly only work with much higher level languages (like Julia and R) and for Data Science purposes. That said I did spend a fair amount of hours reading your code and think this solution is particularly innocuous. As you may attest.

I have also tackled the other problem I had in issue #34: Adding Custom Categories and if you are receptive I am willing to make another pull request. That, however, is a much more consequential change.

[Figuera]

If it comes to modifying categories: please don't.

That's a shame. I am very happy with the results and don't think the necessary code is adding too much complexity.

[nwg-piotr]

Well, this is only needed for your, very specific use case, while I'd like to keep the app as simple, as possible. But of course, show me the code.