Allow adding changelog items using $EDITOR

[jnv]

Current readline support is nice, but you cannot fix typos in lines you already added. Add configuration option #46 to use $EDITOR by default akin to Git's interactive rebase behavior.

[jiripavelka]

It may support multi-line (apart from CLI, see #45) and further editor features.

• Use case 1: typical changelog edit is one-line, use parameter for multi-line
• Use case 2: typical changelog edit is multi-line, use configuration parameter
• Use case 3: requiring editor spellcheck, use configuration parameter