New function for filtering by date

Also keybinding for find-file-at-point

Altered items:
syslog-mode-map, syslog-datetime-regexp, syslog-date-to-time,
syslog-filter-dates, syslog-mode
Alter keybindings, and add new one for dired

Altered items:
syslog-mode-map
New commands syslog-previous-file and syslog-next-file

Also alter keybindings so that highlight commands all use h prefix key.

Altered items:
syslog-mode-map, syslog-previous-file, syslog-next-file
Change default args for syslog-filter-dates

Change initial and default values of start and end dates in
syslog-filter-dates to be first and last dates in buffer.
Also change syslog-date-to-time so that it returns nil if an incorrect
date is given when in safe mode.

Altered items:
syslog-date-to-time, syslog-filter-dates
Add customization group
Add TODO comment