Only show available editors with correct path in list

[simeg]

Background
(#38, #39)
This project is written for macOS (because me, the author, uses a Mac) but not every user uses macOS. On Ubuntu the path to nano is different from macOS.

Proposed solution
Before showing the user the list of editors, we should "query" the system to see what binaries are available at what locations.

Note: If the command which [editor] returns 0 we can 1) know that the binary exists on the system, and 2) we get the path. So which would be a good command for this.

[iamaamir]

which returns the pathnames of the files (or links) and It does this by searching the PATH, you could search the $PATH and then walk the dirs to.

another trick could be just run the command with -v if we got back something meaning command is present.

or the better is to use a crate

[simeg]

Yeah, we could search the PATH directly.

Ideally I wouldn't want to add a dependency for this, but because it's cross-platform I think I'd be OK with it. I have no idea how the rest of this tool works on Windows though.

[iamaamir]

these tools are *nix olny so it would be better to use a crate

[simeg]

Redundant after #36