Allow amending existing manifest

[labaneilers]

Allow, when passing a list of files to process, updating an existing manifest (instead of deleting it and starting from scratch). We could load the manifest into memory, update it, and rewrite it. We could also optimize so as not write files that already exist with correct hashcode, or look at timestamps to avoid even recalculating hashcodes at all (also see issue #18).

We'll need to add functionality to parse a manifest to the manifest format adapters. I'm not sure how this will work with non-self describing adapters, which may have data written from plugins; perhaps we'll just include a list of extra fields, which can be accessed by index. The same plugins would have to be loaded both times.

[labaneilers]

Document with an example of piping in and using amend
find . -name *.png | hashly ./ --amend
probably more useful with something like svn status, or find -mtime:
find . -mtime -1 -type f | hashly ./ --amend