Search field that sends to hoogle

The list of diagrams' packages gets appended to the query.

issue #23

List of appended packages can be modified by changing variable
`PACKAGES` in `web/js/hoogle-search.js`. At the moment it is"

    var PACKAGES = '+diagrams-contrib +diagrams-lib +diagrams-core +diagrams-canvas \
    +diagrams-cairo +diagrams-rasterific +diagrams-svg +diagrams-builder \
    +diagrams-postscript +active +palette +diagrams-constraints'