index: begin scan from specified data file

[jbride]

Could be a nice feature to have gabriel begin its indexing from an optionally specified block data file. ie:

./gabriel index -start-scan-block-file=blk04553.dat

If/when specified, gabriel would begin evaluation from this specified block data file (as opposed to beginning w/ the genesis block file: blk00000.dat ).

semi-related: probably would want to avoid duplicate entries in output data store (currently a csv file indexed by block height).

[cryptoquick]

Good thinking.
Also, at some point I think we're going to need to use a database like sled.