Changes for extension authors

[Bryan O'Sullivan]

I've made a few changes to the way extensions are handled.

The cmdtable dict and reposetup function are now optional, and nothing
will happen if they are not provided.

There's a new hgext directory in the source tree.  Any packages put in
there will be installed with the rest of Mercurial, and can be referred
to in the [extensions] section of a hgrc file as hgext.packagename.

The wiki documentation has been updated to reflect this.

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