feat: prevent timing out one tab or window if another tab have activity

[roupify]

Add a LocalStorageExpiry that put the expiry value in LocalStorage. If localStorage is not supported
by the browser, will store the expiry value in memory.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/HackedByChinese/ng2-idle/blob/master/CONTRIBUTING.md#pr
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[X] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

Multiple tabs or windows wasn't supported.

What is the new behavior?

Now, the expiry value is set in the localStorage by default, and if a browser doesn't support it (i.e. Safari in private mode), we store the expiry value in memory.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[X] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[coveralls]

Coverage decreased (-0.6%) to 98.267% when pulling 2be6a45 on rousseaufiliong:localStorage into a682826 on HackedByChinese:master.

[coveralls]

Coverage decreased (-0.08%) to 98.762% when pulling 50d30c4 on rousseaufiliong:localStorage into a682826 on HackedByChinese:master.

[coveralls]

Coverage increased (+0.2%) to 99.01% when pulling 1f8b29c on rousseaufiliong:localStorage into a682826 on HackedByChinese:master.

[grbsk]

Another thing I learned the hard way from ng-idle was not to be too prescriptive or bundle in anything that wasn't absolutely essential, thus why this rewrite is very modular (perhaps irritatingly so).

Thus, rather than making this the new default and including in /core, should we make it a new module ala keepalive and let the dev decide if they want this? I'm kind of on the fence about this myself, as it's probable that most people would want this rather than SimpleExpiry anyways.

[grbsk]

One thing people have been asking for from ng-idle was the ability to change the key so that if they had multiple Angular apps in one domain (or, as a new challenge now that we can support multiple Idle instances per app) they don't conflict.

We could allow for this by taking in a new constructor parameter key: string (defaulting to the current expiry) and use that when getting or setting localStorage. The developer could then opt-in to this feature by initializing and providing a new instance rather than the current default LocalStorageExpiry.

[roupify]

Great idea, I'll do the changes this morning.

[coveralls]

Coverage increased (+0.4%) to 99.279% when pulling cfd0b15 on rousseaufiliong:localStorage into a682826 on HackedByChinese:master.