Feature/cmd list terms

Hello,

This is a proof-of-concept "list" command: it lists all the stored terms in the mu xapian database, with their type ( body/subject/from/to/uid/tag/) and their frequency.

Example:
```
$ ./mu/mu list
body	0	3
body	0.0	1
body	0.2	1
flag	u	11
msgid	500876e4_9060401_cshl_edu	1
msgid	50087ad7_1010005_cshl_edu	1
subject	and	3
subject	are	1
subject	awk	1
subject	bam	1
uid	1391647acbe22335	1
uid	359911bb72c55736	1
uid	57d140325497494d	1
tag	revcomp	1
tag	reverse	1
tag	sam	1
tag	seq	1
```

This will enable front-ends to build lists for auto-complete and such.

The patch isn't yet production quality, but any comments are welcomed.

-gordon