Add separate changelog keyword

[petrzpav]

This will execute CLI for adding entries to changelog.

Consider adding default aliases like omgf ch or omgf c.

[jnv]

Should this also commit changes to CHANGELOG file?

Options:

• autocommit by default, but allow turning this off with switch,
• make autocommit opt-in with switch switch,
• ask user whether she wants to commit the change,
  • and add switch for Yes or No,
• just stage the changes,
• do nothing.

[jiripavelka]

IMO KISS: always autocommit and keep repairs to user as any other bug. And since it is a separate command, make it one-line.