Remove lodash

[Daniel15]

Would you accept a PR that removes the lodash requirement?

• !isRegExp(re) and isString can be replaced with typeof re === 'string'
• flatten can be replaced with flatMap (as long as IE11 support is not required, but you could specify that a polyfill is needed for IE11 support)
• escapeRegExp can just be inlined

[iansinnott]

Sure thing 👍

Although it seems there's no need to drop IE11 support just for flatMap, since flatten can be rewritten in just a few lines.

[FoxxMD]

Considering lodash is the majority of the package size this would be a huge win!

[chadfawcett]

This can probably be closed now that #71 has been merged.

[Daniel15]

Sorry I didn't get around to submitting a PR for this. Thanks @bianzheCN for doing it :)