add content_type for DSL display
complete DSL rework

  Allow nested dash/graph/metric definitions, and structure reopening
  Fixes #10
  Fixes #8
  Fixes #7
  Fixes #3
Merge pull request #17 from josqu4red/dsl_display

add content_type for DSL display
add checks on dsl parsing
add more error checking in dsl, fix regression on load
Merge pull request #18 from josqu4red/dsl

Complete DSL rework
Merge branch 'master' into new_definitions