* add basic implementation for bookmark support.

basically, add a file named ~/.mu/bookmarks which looks something like
  ----
  [mu]
  testarch=subject:test maildir:/archive
  inbox=maildir:/inbox
  ----

  now, one can do a query, e.g.

    mu find -b testarch

  which is equivalend to

    mu find subject:test maildir:/archive

 if both a bookmark and a 'normal' search expression are specified,
 the will be concatenated.

 bookmarks starting with mu_ are reserved for mu itself.

src/Makefile.am

@@ -46,6 +46,8 @@ mu_LDADD=				\
 	libmu.la
 
 libmu_la_SOURCES=			\
+	mu-bookmarks.c			\
+	mu-bookmarks.h			\
 	mu-cmd-extract.c		\
 	mu-cmd-find.c			\
 	mu-cmd-index.c			\