Does not run on MacVim

This plugin does not run on MacVim. After all node dependecies are available, the browser will be opened but does not display any content.

Steps to reproduce the behavior:
- `brew install macvim`
- `:MarkdownPreview`

System:
```
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled May 26 2020 06:16:34)
macOS version
Included patches: 1-539
Compiled by Homebrew
Huge version with MacVim GUI.
```

Log:

It tries to load the `neovim` node module but it should not because we are on vanilla vim (with MacOS GUI) and we'll see in the messages that it also tries to use it:

```
[winston] Attempt to write logs with no transports {"message":"request -> neovim.api.nvim_get_var","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"request -> neovim.api.nvim_set_var","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"response -> neovim.api.nvim_get_var: 0","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"request -> neovim.api.nvim_get_var","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"response -> neovim.api.nvim_set_var: 0","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"response -> neovim.api.nvim_get_var: ","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"request -> neovim.api.nvim_call_function","level":"debug"}
[winston] Attempt to write logs with no transports {"level":"info","message":"handleNotification: "}
[winston] Attempt to write logs with no transports {"message":"request -> neovim.api.nvim_list_bufs","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"response -> neovim.api.nvim_call_function: 0","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"response -> neovim.api.nvim_list_bufs: [object Object]","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"request -> neovim.api.nvim_get_var","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"response -> neovim.api.nvim_get_var: ","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"request -> neovim.api.nvim_get_var","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"response -> neovim.api.nvim_get_var: ","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"request -> neovim.api.nvim_get_var","level":"debug"}
[winston] Attempt to write logs with no transports {"message":"response -> neovim.api.nvim_get_var: ","level":"debug"}
```