mapsf_0.7.0

Fix

• use a vector of colors for lines if necessary in mf_label() (#50)
• enable pipe without side effects (no extra plot)
• use a default transparent background for insets
• deprecate "theme" arg in relevant function, adapt docs and vignettes, use
  options() for themes instead of global variable & mimic the behaviour of
  basetheme package
• force the use of cairo device, if available, in mf_export() for png
• fix wrong class allocation when using breaks = "jenks" (#53)
• exports using unprojected objects do not produce figures with inaccurate
  height/width ratio anymore.

Feat

• add expandBB arg in mf_map()
• add expandBB arg in mf_raster()
• add arg checking depending on type in mf_map()
• add an error message for functions that need a pre-existing plot
• add mf_graticule() to add graticule lines and labels
• set internally the 'add' arg for each map types in mf_map()