Add support to include whitespace between elements

[steakchaser]

The single tick works fine for adding whitespace after text blocks, but many layouts rely on whitespace between elements. Slim offers a < or > operator that can be used prepend / append whitespace to elements:

a< href='url1' Link1

Would be sweet is emblem had something similar!

[bj-mcduck]

related-ish to #156

[SeanRoberts]

+1

[nybblr]

++1. On an unrelated note, it was the only hurdle I faced transferring a ton of Rails slim views to an Ember app =)

[radicalsauce]

1++

[batamire]

+= 1