Initial tests for non-specific digest (#37)

[lazear]

This PR adds support for the items in https://github.com/lazear/sage/issues/37

• An optional c_terminal field is added to the enzyme struct, that allows N-terminal digestion
• cleave_at can be set to the special character "$" (e.g. end of line for standard regex) to turn off digestion and match the entire fasta entry
• cleave_at can be set to the empty string "" to denote non-specific digestion

Notably, min_len, max_len and missed_cleavages still apply to all of the above (however, restrict and c_terminal are ignored for non-specific or no- digest) - the following configuration will produce all possible peptides between 5 and 7 AA's long:

    "enzyme": {
      "missed_cleavages": 0, 
      "min_len": 5, 
      "max_len": 7,
      "cleave_at": "", 
      "restrict": "P" 
      "c_terminal": true
    }