Allow editing blocks (or even pages) in external editor

[agentydragon]

Athens is not a fully-featured text editor, compared to editors like Vim or Emacs. No syntax highlighting while editing, no autocompletion, no macros, etc.
Other features I miss are things like search & replace, moving by whole words, etc.
If we allow people to use a text editor they're experienced with, we don't have to invent our own.

Allow editing blocks in external editor

I suggest that we implement a shortcut (e.g., Ctrl+Alt+X or something). When you click the shortcut, Athens creates a temporary file with the content of the current block, opens the block with an editor of the user's choice (set in settings), and waits until the editor exits. Until the editor exits, Athens does not allow editing the block. When the editor exits, read the temporary file (that the editor overwrote), and replace the block's content with the tempfile's content.

if it would make it easier, would be also fine for Athens to just freeze until the external editor closes.

Advanced version: allow editing pages in external editor

e.g., start with this Athens page:

# Hello
* content of first block
* content of second block

when opened in editor, this tempfile gets created

# Hello
(123abc) content of first block
(abc123) content of second block

user changes to:

# Hello
(123abc) content of first block
(abc123) overwritten content of second block
(new) new third block

Athens knows that block 2 should be overwritten because ID remained the same, and knows to add a new block for the third one.