This adds support for the -m flag (print metadata) so that rule metadata is
printed when a scan matches. Currently only outputs in text form, json will be
next.

If a string is using the xor modifier we now display the xor information (key
and plaintext) in both text and json output modes.

```
wxs@mbp yara-x % ./target/debug/yr scan -o ndjson -s rules/a.yara ~/src/yara/tests/data/xor.out | jq .
{
  "path": "/Users/wxs/src/yara/tests/data/xor.out",
  "rules": [
    {
      "identifier": "freebsd",
      "strings": [
        {
          "identifier": "$a",
          "start": 28,
          "length": 19,
          "data": "Uihr!qsnfs`l!b`oonu",
          "xor_key": 1,
          "plaintext": "This program cannot"
        },
        {
          "identifier": "$a",
          "start": 52,
          "length": 19,
          "data": "Vjkq\\\"rpmepco\\\"acllmv",
          "xor_key": 2,
          "plaintext": "This program cannot"
        },
        {
          "identifier": "$b",
          "start": 4,
          "length": 19,
          "data": "This program cannot"
        }
      ]
    }
  ]
}
wxs@mbp yara-x % ./target/debug/yr scan -s rules/a.yara ~/src/yara/tests/data/xor.out
freebsd /Users/wxs/src/yara/tests/data/xor.out
0x1c:19:$a xor(0x1,This program cannot): Uihr!qsnfs`l!b`oonu
0x34:19:$a xor(0x2,This program cannot): Vjkq\"rpmepco\"acllmv
0x4:19:$b: This program cannot
────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 1 file(s) scanned in 0.0s. 1 file(s) matched.
wxs@mbp yara-x %
```

When using --print-strings-limit it looks like this in text mode:

```
wxs@mbp yara-x % ./target/debug/yr scan -s --print-strings-limit 5 rules/a.yara ~/src/yara/tests/data/xor.out
freebsd /Users/wxs/src/yara/tests/data/xor.out
0x1c:19:$a xor(0x1,This ): Uihr! ... 14 more bytes
0x34:19:$a xor(0x2,This ): Vjkq\" ... 14 more bytes
0x4:19:$b: This  ... 14 more bytes
────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 1 file(s) scanned in 0.0s. 1 file(s) matched.
wxs@mbp yara-x %
```

Not sure if we want to print the "... X more bytes" part in the plaintext or
just leave it implied.

I've also included a bug fix here where we were only printing the last matching
pattern.