Add/edit card creation using external editor (a/e)

[samuelstranges]

Feature request

Context

Issue #5 previously implemented a card creation/editor, but was removed in architectural reset. Currently, users must create cards outside of the application.

Proposed solution

Add ability to create new cards and edit existing cards using external editor ($EDITOR)

• press 'a' while studying a deck to add a card (and add title)
• press 'e' while studying a deck to edit currently selected card
• Launches $EDITOR with a markdown template
• Saves the new card to selected deck after editing

Benefits

• Leverages familiar text editors
• Continues with the file-based philosophy of GoCard

Implementation

I'd love to give this a crack if the approach is approved.

[DavidMiserak]

@samuelstranges Thanks for bringing up the issue. Feel free to fork and submit a pull request.