wp-now: Expose PHP error log

[adamziel]

When developing with wp-now, it would be ideal to have the PHP errors available either in the terminal or in an error_log file/

The suggested wp-config.php settings would be:

define( 'WP_DEBUG', true ); // enable debugging
define( 'WP_DEBUG_DISPLAY', false ); // prevent errors/warnings from displaying on screen
define( 'WP_DEBUG_LOG', '/path/to/active/folder/debug.log' ); // log to errors/warnings to the debug.log file in the open folder

Bonus points if it could be possible to enable/disable the WP_DEBUG_DISPLAY constant, in case a developer wanted to do so.

This is just like #27, but for wp-now

[ktmn]

I'm just trying out wp-now for the first time, I'm following this:

playground-tools/packages/wp-now/README.md

Lines 149 to 168 in 9928f5a

	### Defining Debugging Constants
	In the similar way we can define `WP_DEBUG` constants and read the debug logs.
	Run `wp-now start --blueprint=path/to/blueprint-example.json` where `blueprint-example.json` is:
	```json
	{
	"steps": [
	{
	"step": "defineWpConfigConsts",
	"consts": {
	"WP_DEBUG": true,
	"WP_DEBUG_LOG": true
	},
	"method": "define-before-run"
	}
	]
	}
	```

but I get:

Warning: Constant WP_DEBUG already defined in /var/www/html/wp-config.php on line 82

when actually using that blueprint.

So how is the defineWpConfigConsts step supposed to work when wp-config.php has WP_DEBUG hard-coded?

Edit: just noticed #135