Way to show attributes

[takluyver]

It should be possible to see attributes both in the terminal and in HTML. To figure out:

• In the terminal, this should probably be an option, e.g. --attrs, so they're not cluttering up the view of the default structure.
  • Should the default view show any indication that there are attributes?
  • Should viewing attrs for a file/group replace the tree view? Or still show the tree, with or without attrs for children?
• In the HTML view, how to show attributes without cluttering up the tree view too much?
• Should large attributes be elided somehow?

[takluyver]

The terminal can now show attributes (a few commits around e2dcbc2)

[takluyver]

Leaving this open to think about:

• Showing attributes in the HTML view (I'm focusing on the terminal interface at the moment)
• Showing the full value of long attributes. E.g. European XFEL uses some long mulitline string attributes holding XML-like snippets. There's currently no way to see more than a few characters at the start and end of these.