Allow for script to exit with code 0 even if circular dependencies are found

[louisscruz]

In some cases, one might aim to use madge to get a list of circular dependencies without necessarily exiting 1 in the case that some are found. This PR adds an --allow-circular flag that will allow for that.

[louisscruz]

@pahen would you consider adding this? Or, perhaps some variation of this?

[PabloLION]

liking

• Should allow non-zero exit code when generating graph #294

[PabloLION]

Suggested change

	if (circular.length && !program.allowFailures) {
	if (!program.allowFailures && circular.length) {

Kinda niche-picking but in this way it express better which is more important

[PabloLION]

Suggested change

	.option('--allow-circular', 'exit with code 0 when circular dependencies are found', false)
	.option('--allow-circular', 'not exit with code 1 when circular dependencies are found', false)

Describe what's done instead of using its converse. The old description is ambiguous: imagine the case where circular dependencies are found along with other errors. By the old description, madge was meant to exit with 0, which is untrue.

[PabloLION]

Hi, @louisscruz , thanks for this handy idea. I'm here to help with the PR.
Apart from the aforementioned problems, it seems to me the word "allow" is a bit of vague in the sense of "who allows what": madge won't forbid user to have circular dependency, it only throws error on checking.
Maybe we can consider using word like suppress or ignore?

[PabloLION]

Due to the inactivity, I'm going to do the edit myself and add some output test.