#25786 Adds new parameter 'defer_keyword'

This adds a new manifest parameter called 'defer_keyword'. Here is the
manifest description:

Advanced parameter. This allows for advanced customization around
deferred folder creation. Deferred folder creation allows for the
creation of partial subfolder structures depending on a specific keyword
(see main documentation for details). Before an app is launched, folders
are automatically created and by default (e.g. if you leave this setting
as null), the launch app will pass the name of the engine as the
deferred folder creation keyword. This makes it easy to set up deferred
rules in your folder creation config for tk-maya, tk-nuke etc. However,
if you for example wanted to set up specific deferred folder structures 
for Nuke and Nuke X (both running the nuke engine), you need a finer
granularity. This setting can then be used to override the default
behaviour of just passing the engine name. Instead, you can pass any
string into the deferred folder creation (for example 'nuke' and
'nuke_x' in the case above). This parameter is also useful if you want
to use deferred folder creation in conjunction with launching of apps
which do not have a toolkit engine defined - for these app launch
instances, the engine setting is left blank, and therefore no engine
name is passed into the deferred folder creation. In such cases you can
utilize this parameter to control the deferred folder creation.

Closes #9.