Detect file changes and reload

[MaanooAk]

Based on #86 with some additions.

Operations

• Open an markdown file -> start watching

and also store the watcher so it can be stopped

• Detected file changes -> If the relevant file has not been saved in editor, give user a prompt to reload it, otherwise reload it automatically.

and flag it saved

• Close a tab or quit the app -> stop watching relevant files.

• When file saved -> pause watching until the file is saved

Reload message

{
  type: 'question',
  title: 'EME',
  message: `The ${filename} has been modyfied.`,
  detail: 'Your changes will be lost if you reload the file.',
  buttons: ['Reload', 'Don\'t Reload']
}