allow extra app dependencies for yaws.app to be configured

Yaws can depend on crypto for random numbers and hashes, and on the
compiler for .yaws pages. But the default yaws.app file specifies only
kernel and stdlib as application dependencies, since not all Yaws
deployments use crypto or compiler. Still, if a Yaws system uses crypto and
compiler without including them in the yaws.app file, building and
deploying an Erlang/OTP release for that system will fail.

Add --enable-crypto and --enable-compiler options to the configure script
to allow the user to add either or both of these applications to the
yaws.app file. Specifying the --enable-crypto option also causes the Yaws
startup code to start the crypto application; likewise for
--enable-compiler and the compiler application.

For rebar builds, enhance rebar.config.script to generate the same files
the configure script generates for any extra app dependencies. Setting the
YAWS_APPDEPS environment variable to a space- or comma-separated list of
addition applications to add to yaws.app, e.g.

YAWS_APPDEPS='crypto compiler'

causes rebar to add those apps to the generated yaws.app file and ensure
they're started before Yaws started.