\version "2.19.20"

#(define-markup-command
  (agogical layout props text)
  (markup?)
  (interpret-markup layout props
    (markup #:italic text)))
agogicMark =
#(define-music-function (parser location text) (markup?)
   #{ \tempo \markup \agogical $text #})

rit = \agogicMark "rit."
pocoRit = \agogicMark "poco rit."
accel = \agogicMark "accel."
accelMolto = \agogicMark "accel. molto"
aTempo = \agogicMark "a tempo."