Warning message added before executing image prune
Added a force option, to execute without user input.

Signed-off-by: Kunal Kushwaha <kunal.kushwaha@gmail.com>

timetype package include functions for calculating
time input in string as one of golang duration format and
calculate as unix timestamp

Signed-off-by: Kunal Kushwaha <kunal.kushwaha@gmail.com>

filter option accepts two filters.
- label
- until
label supports "label=value" or "label=key=value" format
until supports all golang compatible time/duration formats.

Signed-off-by: Kunal Kushwaha <kunal.kushwaha@gmail.com>

examples of image prune with filter and
until options added

Signed-off-by: Kunal Kushwaha <kunal.kushwaha@gmail.com>