Add TOML rule for triple quotes

**Is your feature request related to a problem? Please describe.**
TOML uses the same types of triple quote strings that Python does, but this plugin does not have such a TOML rule.

**Describe the solution you'd like**
Add a rule to handle triple quote strings for TOML, just like Python does.

**Describe alternatives you've considered**
Manually add these rules in my vimrc.
