ipset: Monitor set size

### Use Case

I don't want to monitor each entry in my ipset, but I want to monitor the whole ipset size, because I automatically add new items to it.

### Expected behavior

The current metric should be optional (e.g. `gather_entries` like in mysql). Furthermore, there should be an option to gather the whole set size using the `ipset list` command.

### Actual behavior

Only monitoring the entries via `ipset save` is possible.

### Additional info

Example commands:

```
$ sudo ipset save
create myset hash:net family inet hashsize 1024 maxelem 65536 counters comment
add myset 10.69.152.1 packets 0 bytes 0
```

→ existing use case, results in metric like `ipset,rule=10.69.152.1,set=myset bytes_total=0i,packets_total=0i 1507615028000000000`

```
$ sudo ipset list
Name: myset
Type: hash:net
Revision: 6
Header: family inet hashsize 1024 maxelem 65536 counters comment
Size in memory: 552
References: 0
Number of entries: 1
Members:
10.69.152.1 packets 0 bytes 0
```

→ new use case, results in metric like `ipset_size,set=myset entries=1i,memory_size=552i 1507615028000000000`

Start with Telegraf's ipset input plugin and its existing ipset save collection path. Compare the requested fields from the ipset list command with the current per-entry metrics, and determine how an option such as gather_entries should control them. Done means set-level size metrics can be collected while entry metrics remain optional, matching the example outputs.

Written by the indexing model from the issue text.