Add new custom variable 'projectile-cmd-hist-ignoredups'

[LaurenceWarne]

Hi! This PR adds new custom variable projectile-cmd-hist-ignoredups, which can be used to tweak how duplicates are dealt with in projectile's command history. The custom variable is more or less identical in behaviour to eshell-hist-ignoredups.

Specifically, the existing default behavior is maintained with the value of t, which means consecutive duplicates are ignored. A value of 'erase means only the last duplicate is kept, whilst a value of nil means all duplicates are kept.

(PR is an iteration on https://github.com/bbatsov/projectile/pull/1774)

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Thanks!

[bbatsov]

The proposed change is fine by me. Might be a good idea to mention this configuration option somewhere in the docs as well, otherwise I doubt many people will find it.

[LaurenceWarne]

The proposed change is fine by me. Might be a good idea to mention this configuration option somewhere in the docs as well, otherwise I doubt many people will find it.

Makes sense, let me know if the section I've added it looks appropriate.

[bbatsov]

Looks great! Thanks!

[LaurenceWarne]

Thanks!