startup hooks

[FND]

TiddlyWiki should provide hooks for the four startup phases in order to provide greater flexibility for plugins and to promote cleaner code.

Reasoning

Currently, plugins need to hijack restart() for initialization purposes.

The only workaround is to create a macro and use its initialization function (e.g. config.macros.foo.init()). However, this should not be encouraged as it pollutes the macros namespace.

[FND]

cf. #978