format of --annotation snp input

[CarolineOhrman]

It would be convenient to be able to comment out lines in the snp file used as input for building the database and also to have a commentfield.

Proposed format

snp_id	strain	reference	genome	position	derived_base	ancestral_base	comment
B.1	francisella	Birdsell2014	OSU18	1710718	T	C	this is an optional free text comment field
B.2	francisella	Vogler2009	OSU18	19938	T	G
B.3	francisella	Vogler2009	OSU18	1540403	T	C
#B4 is one of the major branches
B.4	francisella	Vogler2009	OSU18	1350634	A	T	position in SCHUS4 XXXX
B.5	francisella	Vogler2009	OSU18	84943	T	C
#B6 is one of the major branches
B.6	francisella	Vogler2009	OSU18	1471420	T	C
...