Extend LEIN_SILENT to cljsbuild (#417)

[tmarble]

Per the comment in #417 I have extended the LEIN_SILENT support to lein-cljsbuild.

Please note this introduces an additional, explicit dependency of cljsbuild on [leiningen-core "2.5.3"] which may be problematic (the proposed delta for lein-cljsbuild doesn't change the dependencies due to :eval-in-leiningen true).

NOTE: there remains this one non-Warning and non-Error println not converted (but perhaps should be?)

• https://github.com/emezeske/lein-cljsbuild/blob/master/support/src/cljsbuild/test.clj#L17

NOTE: There are whitespace changes in support/project.clj (due to clojure-mode and my Emacs (add-hook 'before-save-hook 'delete-trailing-whitespace)). The other significant change is change in scope of org.clojure/clojure to "provided".

[danielcompton]

This PR also adds a dependency on Leiningen in the support plugin. While it may or may not be an issue for the Leiningen plugin itself, I suspect this dependency will be unwanted by any other project that wants to use cljsbuild in a programmatic fashion. There are also other usages of println used throughout the support project which haven't been converted yet?

[tmarble]

Right, hence my comments above.

One could argue that we add a hack like this at the top of compiler.clj, but I'm not sure it's more elegant:

(let [lmain-info (ns-resolve (the-ns 'leiningen.core.main) 'info)]
  (def info (or lmain-info println)))

[mneise]

I agree with @danielcompton that there is quite a bit of overlap with #419 which would make this PR redundant. The approach of having default loggers in #419 seems like a good idea which would save us from adding an additional dependency to the support project.