Changes are not taken when putting a "/" character in the filename

[romainnorberg]

Environment info

• OS: MacOs Sierra 10.12.6 (16G1212)
• App Version (Command/Ctrl + ,): v1.6.0

Description of the problem / feature request / question:

Changes are not taken when putting a "/" character in the filename (see gif), no error message is provided.
Github doesn't allow /' in the name

If possible, provide a sample gist or screenshot:

[hackjutsu]

It's not a valid token for a filename in most OSs, maybe we need a warning for that. To be honest, I don't know why users would try this...

[romainnorberg]

users often do mistake like this

[hackjutsu]

FYI, cmd + e is the shortcut for editing an existing gist.

[hackjutsu]

Duplicate issue: #268
Reference package: https://www.npmjs.com/package/valid-filename
Relevant code: https://github.com/hackjutsu/Lepton/blob/master/app/containers/gistEditorForm/index.js#L68

This seems to be a simple fix and a low hanging PR.