Dev mode detect

[maartenbreddels]

• Closes Detect if we are in a repo #135.
• Needs a rebase after Give an error when a template is not found #137
• Includes prettier printing of search paths:

[Voila] using template: gridstack
[Voila] nbconvert template paths (\t\n) instead of comma separated lists:
	/Users/maartenbreddels/src/QuantStack/voila/share/jupyter/voila/template/gridstack/nbconvert_templates
	/Users/maartenbreddels/src/QuantStack/voila/share/jupyter/voila/template/default/nbconvert_templates
[Voila] template paths:
	/Users/maartenbreddels/src/QuantStack/voila/share/jupyter/voila/template/gridstack/templates
	/Users/maartenbreddels/src/QuantStack/voila/share/jupyter/voila/template/default/templates
[Voila] static paths:
	/Users/maartenbreddels/src/QuantStack/voila/share/jupyter/voila/template/gridstack/static
	/Users/maartenbreddels/src/QuantStack/voila/share/jupyter/voila/template/default/static
	/Users/maartenbreddels/src/QuantStack/voila/voila/static

[maartenbreddels]

@SylvainCorlay could you take a look, if you agree with https://github.com/QuantStack/voila/pull/138/files#diff-5c1fed614067afed13da288c9daf0d20R16
we can merge this.

[SylvainCorlay]

Sorry, I though I had merged this already!