Enhancement: allow for custom bracket types

[d3v-null]

Hey there, loving the plugin, it's a good drop in replacement for the bracket selection functionality i'm used to from vim surround , however I would love to be able to extend the brackets that are allowed to be matched. e.g. when I'm in writing my notes in markdown, I like to be able to select the inner content of code blocks that are delimited by ``` and ``` it would be nice if one could configure what bracket pairs the plugin looks for from settings. If I find some spare time would you accept a pull request? Thanks,