Use vim functions instead of system()

[michelesr]

Fix #51 #53

Using vim functions instead of external programs dramatically reduces the delay when opening files. Tested with neovim and vim.

In order to make it work with vim I had to add the _close_prev parameter (prefixed with underscore as unused), otherwise calling the function will cause the error too many arguments passed.

I appreciate your comments on #4 about the fact that anyone want a personal version, but this PR is just an improvement and is not changing the logic (at least for what I can see).

Please let me know your feedback.

[francoiscabrol]

please use the params self.edit_cmd instead of 'edit ' to keep the "open the file in a new tab" feature working

[francoiscabrol]

Thank you for your contribution! I will try locally

[michelesr]

@francoiscabrol: I've updated the code to use self.edit_cmd in the nvim part (a:edit_cmd must be used for the vim part because we're not inside a dictionary based function so self is not available)

[francoiscabrol]

I found one weird thing: with your changes, if I try to open a folder with nvm . and then I open a file, the file is not open. Well it is open but also close right after.
If you remove

      if self.close_prev == 1
        exec "bp"
        exec "bd!"
      endif

that fix the issue.
These lines were added recently with #50 to fix an issue that does not exist anymore with your implementation.
So if you remove them, then your PR will be ready.

[francoiscabrol]

This PR should fix also the issue #53

[michelesr]

I removed the parameter, now it should be all done.

[francoiscabrol]

awesome! I tried and everything looks good. Thank you!