Support custom column name for some methods

[meness]

Hi,

Changing the default deleted_at, created_at, and updated_at column names is possible by creating new constants called DELETED_AT, CREATED_AT, and UPDATED_AT.

$table->softDeletes(); // creates 'deleted_at' column

$table->softDeletes('left_at'); // creates 'left_at' column

Affected methods: softDeletesTz(), timestampsTz(), timestamps(), dropSoftDeletes(), dropTimestamps()

[taylorotwell]

I dunno. I don't want to go overboard with this stuff. You can just call the methods you need directly.

[meness]

@taylorotwell You already merged #19203. This PR exactly does what #19203 does. You made helper functions but you forced users to use them only if they have exact deleted_at column, is this right?!