feat: add time in labels

[zkoppert]

fixes #41

Adds option to specify a list of LABELS_TO_MEASURE. ie. LABELS_TO_MEASURE = "bug,enhancement,bogus"
Output then adds time spent in label and the average. ie (see below)

Issue Metrics

Metric	Value
Average time to first response	4 days, 6:28:54.625000
Average time to close	15 days, 3:41:37.090909
Average time to answer	None
Average time spent in bug	49 days, 1:49:59
Average time spent in enhancement	46 days, 0:39:46
Average time spent in bogus	None
Number of items that remain open	0
Number of items closed	11
Total number of items created	11

Title	URL	Time to first response	Time to close	Time to answer	Time spent in bug	Time spent in enhancement	Time spent in bogus
Failed to deploy to production	super-linter/super-linter#4224	0:02:51	0:02:51	None	None	None	None
Failed to deploy to production	super-linter/super-linter#4223	0:03:40	0:03:40	None	None	None	None
Support an array of default files for each linter, based on that linters expectations.	super-linter/super-linter#4222	1:39:58	46 days, 0:40:31	None	None	46 days, 0:39:46	None
Failed to deploy to production	super-linter/super-linter#4221	0:29:38	0:29:38	None	None	None	None
Failed to deploy to production	super-linter/super-linter#4220	0:29:30	0:29:30	None	None	None	None
[BUG]	super-linter/super-linter#4207	None	2 days, 22:30:38	None	None	None	None
Vivox80pro	super-linter/super-linter#4206	None	3 days, 2:14:08	None	None	None	None
1e+300	super-linter/super-linter#4205	None	3 days, 2:14:50	None	None	None	None
Failed to deploy to production	super-linter/super-linter#4204	1:30:22	1:30:22	None	None	None	None
Failed to deploy to production	super-linter/super-linter#4203	7 days, 5:48:59	49 days, 3:24:45	None	49 days, 1:49:59	None	None
pulling the super linter docker image is taking long time even if we use slim image (around one and half min). tried caching the image but size of the image is too long and for restoring the image also taking long time. Is there any way to improve the time?	super-linter/super-linter#4186	26 days, 17:46:19	62 days, 6:56:55	None	None	None	None