refactor generate-combined.sh

Update the generate-combined.sh script to support printing an ordered
source file list, cat'ing the compressed or uncompressed output, or
updating the dygraph-combined.js file (the default). This is intended to
make it easier to integrate dygraphs code into other builds.

Move the plugins/{base,install}.js files to
dygraph-plugin-{base,install}.js, this simplifies the file list
by permitting use of plugins/*.js with no order dependency.