Allow to omit large common parts in diff output

[njam]

By default disabled, can be enabled and configured with --context NUM.

Wdyt?
This is my first time writing Go code, please let me know if my naming is off, something should be refactored or just be done differently!

The code iterates over all diff records and keep track of how many lines are between the closest change (addition or removal) before the current line. Then it iterates again through all diff records but backwards and adjusts the distance if the closest change after the current line is closer.
When printing the diff we can then simply omit records that have a distance larger than the desired context.

I have an alternative implementation which first creates chunks and then iterates over chunks of diffs to print, if you prefer it: diff-with-chunks.go.

[njam]

@databus23 I've updated the branch to the new CLI interace.
Maybe a flag like --context which is not available in helm upgrade should go before the "upgrade" now? So it's helm diff --context 3 upgrade ...?

[helgi]

This one would be great to have, even without --context or similar; makes it similar to have diff works

[mumoshu]

@njam Thank you so much for contributing the awesome feature!

@zytek updated your work in #53 and I added my comments there. Would you mind taking a look?

[njam]

@mumoshu I've added the --context option to all CLI commands that print a diff, and added a unit test.
wdyt?

[njam]

@mumoshu when you have a moment, would you be able to review this?

[njam]

@mumoshu let me know if something should be changed! :)

[mumoshu]

@njam Thanks for your effort and the reminder! LGTM.

[njam]

Cool, thanks!

[njam]

Should I add a Travis config? Or you will do it?