Add CLI flag to load-into-counting.py allowing TSV output of basic stats

Hi all,

We're developing a few things based off khmer, and one of the metrics we're using is the total number of k-mers. I know this can be obtained programmatically via the khmer API (by loading in the counting hash and grabbing its occupancy), but this takes a while with many large hashes. So, currently, we're grepping through log files and .info files for the FPR and total number of kmers. 

This PR adds a CLI flag to load-into-counting.py allowing it to output the FPR, total number of distinct k-mers (cm-sketch occupancy) and input files in a machine/R/spreadsheet readable format (tab-seperated file). This saves us from resorting to regex hacks to pull this info out of out.kh.info and logs. I've added a test of the tsv creation behaviour too, and pep8 is clean.

An example of the tsv format is below (markdown table-ified for prettyness):

```bash
python ./scripts/load-into-counting.py  --machine-readable-info -x 1e7 -N 4 -k 20 tst data/100k-filtered.fa data/100k-surrendered.fa 
```

| name | fpr   | kmers   | files                                          |
|------|-------|---------|------------------------------------------------|
| tst  | 0.010 | 3167409 | data/100k-filtered.fa;data/100k-surrendered.fa |

I've also make the info files contain all files used as input. (This can be reverted, I forgot to separate the commits)

Hope you find it helpful :smile: 

Cheers,
Kevin

Breaking news: see https://github.com/ged-lab/khmer/issues/562#issuecomment-62048392. A new n_kmers() function is pending.