Support generating individual URLs

[simonhamp]

Edit by @jesseleite: We decided to pass urls as args instead... ie. php please ssg:generate /about /contact

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Occasionally, you just want to build a specific page or a set of pages. With this new --url option you can do just that:

php please ssg:generate --url=/about --url=/contact

This will attempt to generate only these two pages and no more. It also doesn't clear the entire build directory

[jesseleite]

Love this feature PR, thanks! ❤️

We discussed, and I reworked to pass in as args instead of requiring --url option flags...

Which is nice and clean in practice...

Just going to add a some tests here once #140 gets merged, as I added some basic generator test coverage over there 👍

[simonhamp]

Agreed, much nicer!