Fix filemanager mouse click

[omarelladen]

Mouse click was broken:

Plugin filemanager: filemanager:1090: attempt to index a non-table object(nil)
stack traceback:
	filemanager:1090: in main chunk
	[G]: ?

Press enter to continue

Fixes #25
Closes zyedidia/micro#3951

[JoeKar]

Seems to work as intended.
We should update the version number and add a new version entry to:
repo.json#L42

[JoeKar]

We should update the version number and add a new version entry to:
repo.json#L42

Realized, that the whole binary/archive handling of different plugins within one updated-plugins repository is crap. We can't simply tag one plugin and use GitHubs automatism to create archives of this one tag, which we can simply link in the repo.json.
Right now every plugin must be archived/zipped and uploaded manually to the assets of the v1.0.0 release.

Long story short:
We should extract every of the updated plugins into its own repository within the micro-editor organization. This makes the single plugin handling much easier.

@dmaluka:
Besides of that do you have any complaints to this PR?

[omarelladen]

Instead of calling directly try_open_at_y(tree_view.Cursor.Loc.Y) in both Tab and mouse click, I added a call to try_open_at_cursor. This function was was unsused and mentioned in the README. I had to remove the 2nd condition of the "if" bc otherwise clicking on ".." in empty directories will not work.