Add a `reaction` command

[kalekundert]

I've been thinking about this for a while, but the trouble is that I need a good syntax for specifying the reagents. One possibility:

• Each reagents is a single argument; colons/semi-colons are used to separate fields:

  sw reaction 'nuclease-free water::to 10 µL' 'RNase H buffer:10x:1 µL' 'RNase H:5000 U/mL:0.2 µL' -n 8