Add importantdiagramonly option to the crepbook document class

The importonlydiagramonly option allows to print only chord diagrams
that have been flagged as important by the author of the song. The
important chords are generally complicated uncommon chords and their
diagram is registered with the starred version of the gtab macro.

tex/crepbook.cls

@@ -10,12 +10,14 @@
 \newif{\iftabs}
 \newif{\iflilypond}
 \newif{\ifnodiagram}
+\newif{\ifimportantdiagramonly}
 \newif{\ifpictures}
 
 \tabsfalse
 \lilypondfalse
 \nodiagramtrue
 \picturesfalse
+\importantdiagramonlyfalse
 
 % Options
 \DeclareOption{tabs}{\tabstrue}
@@ -24,6 +26,7 @@
 \DeclareOption{nolilypond}{\lilypondfalse}
 \DeclareOption{nodiagram}{\nodiagramtrue}
 \DeclareOption{diagram}{\nodiagramfalse}
+\DeclareOption{importantdiagramonly}{\importantdiagramonlytrue\nodiagramfalse}
 \DeclareOption{pictures}{\picturestrue}
 \DeclareOption{nopictures}{\picturesfalse}
 
@@ -217,8 +220,25 @@
   \fi%
 }%
 
+\let\gtab@Original\gtab
+\newcommand{\gtab@Hidden}[2]{}%
 \ifnodiagram%
-  \renewcommand{\gtab}[2]{}%
+\renewcommand{\gtab}{\@ifstar
+  \gtab@Hidden%
+  \gtab@Hidden%
+}
+\else%
+\ifimportantdiagramonly%
+\renewcommand{\gtab}{\@ifstar
+  \gtab@Original%
+  \gtab@Hidden%
+}
+\else%
+\renewcommand{\gtab}{\@ifstar
+  \gtab@Original%
+  \gtab@Original%
+}
+\fi%
 \fi%
 
 \newcommand{\lilypond}[1]{%