Don't duplicate id/created_at/updated_at fields if they are passed in

[EspadaV8]

The current migrations assume that everyone wants the default id and
timestamp definitions added, but this isn't always the case. It now
checks to see if any schema definitions have been passed in for id,
created_at and updated_at and if they have then the default fields
aren't added.

[EspadaV8]

My use case for this is using PostgreSQL. My default is to use uuid columns for ids and timestampTz for the created_at and updated_at columns.

The current behaviour remains for ids if the user doesn't pass in an id definition. The timestamps won't be added if either created_at or updated_at have been passed in (otherwise they'd be a column clash).

Tests have been added for the new functionality.

[tabacitu]

Thank you for taking the time to submit this @EspadaV8 ! Let me know if you're still using the package and would like to take a second look at this PR, before we merge it into v2 #170

This sounds like a great idea so I'll take another look at it after I get to the bottom of the PR&issues stack. It's finally time for this repo to get the attention it deserves. Personally I've been using it without adding improvements for YEARS now, I think it's finally time to bring it closer to its potential.

Let me know if you want to help out with another look at this, or with other features/improvements for v2 - we could use all the help we can get!

Cheers!

[EspadaV8]

Haha, yes, this is a bit old now. Looking at the code is still seems okay. Not sure what changes you have planned for the package now but feel free to merge this in if it's useful. I actually stopped using the package because of this (and other) issue(s) and ended up writing some custom Artisan commands with custom stub files.