bookmarklet: use selected text as description when adding a new link

 * Based on romnGit's work at https://github.com/sebsauvage/Shaarli/pull/104
 * Fixes https://github.com/shaarli/Shaarli/issues/18
 * Closes https://github.com/sebsauvage/Shaarli/pull/104
 * Fixes https://github.com/sebsauvage/Shaarli/issues/53
 * Fixes https://github.com/sebsauvage/Shaarli/issues/129
 * Fixes https://github.com/sebsauvage/Shaarli/issues/33

Selected text on a page will be used to fill the "Description" field in the "Add link" dialog when using the bookmarklet to save a link.