Add a tail parameter to pod collector.

[porridge]

What this PR does / why we need it:

Ability to set the number of lines to collect.
Also minor improvements to contributor guide.

Fixes #216

[kensipe]

love it! thanks!

the project has a dependency on testify... could you switch out the newly added dependency.. then we are good!

[kensipe]

briefly looked into this... I see you are getting DeepEqual... is there a way to do this with our current tooling?

[kensipe]

value of DeepEquals

=== RUN   TestPodCommand/pod_name_with_set_tail
    collector_test.go:157: assertion failed: 
        --- cmd.Command
        +++ tt.cmd
          strings.Join({
          	"kubectl logs --prefix foo -n $NAMESPACE --all-containers --tail=",
          	"4",
        - 	"2",
        + 	"3",
          }, "")
        
--- FAIL: TestPodCommand (0.00s)

value of testify .Equal

=== RUN   TestPodCommand/pod_name_with_set_tail
    collector_test.go:157: 
        	Error Trace:	collector_test.go:157
        	Error:      	Not equal: 
        	            	expected: "kubectl logs --prefix foo -n $NAMESPACE --all-containers --tail=42"
        	            	actual  : "kubectl logs --prefix foo -n $NAMESPACE --all-containers --tail=43"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-kubectl logs --prefix foo -n $NAMESPACE --all-containers --tail=42
        	            	+kubectl logs --prefix foo -n $NAMESPACE --all-containers --tail=43
        	Test:       	TestPodCommand/pod_name_with_set_tail
--- FAIL: TestPodCommand (0.00s)

[porridge]

@kensipe I switched to testify and mentioned it in the contributor guide along with a couple other minor improvements there, PTAL.

[kensipe]

fantastic! nice work!