Decorated line (-Sq) doesn't work if the label contains colons

_Originally reported in PyGMT:_ https://github.com/GenericMappingTools/pygmt/pull/664#discussion_r511395136

**Full script that generated the error**

```
gmt plot -R0/10/0/10 -JX10c -Sqn1:+lA:B -W2p,tomato,4_2:2p -pdf map << EOF
0  5
10 5
EOF
```

**Output:**

![image](https://user-images.githubusercontent.com/3974108/97092965-350bf600-1616-11eb-916a-34bb20c42594.png)