Enhancement: set a default split time

[heikkil]

Use defcustom to set a time, e.g. "1h" that is suggested as the default time string argument for the org-clock-split function. Display that time at the prompt and use it if no value is given. If a new value is given, store it and use as the suggested value from there on.

This would allow for quicker splitting of a long time range into chunks of time of known time, e.g. class.