List directories being watched when server is run

List directories being watched when server is run

Fixes part of issue 1030. Previously hugo only listed the content
directory as being watched. Now we list all files being watched
according to `commands.getDirList()`. We also introduce a RemoveSubpaths
function and test in the helpers module to reduce noise in the command
line output by not showing subdirectories of ones already being watched.

For example, instead of:
`Watching for changes in $HOME/blog/content`
We get:
`Watching for changes in
$HOME/blog/{data,content,layouts,static,themes/my-theme}`