Add ability to sort changelog entries alphabetically

[thanethomson]

At present, individual changelog entries are sorted by way of their issue/PR number, but this can result in somewhat chaotic-looking changelogs if entries are prefixed by, for example, a module name (example).

A new configuration file parameter should be provided to allow for configuration of this value:

# The default behaviour
sort_entries_by = "id"

# Sort entries alphabetically by way of the entry text
sort_entries_by = "entry-text"