add dotAll regexp flag

[fujisawa]

This flag is useful especially when it's used with replace function. Let's say we have a big string that contains html and we want to extract the title from it. If we have dotAll regexp flag, we can write the code like this:

replace (unsafeRegex "^.*<title>([^<]+)</title>.*$" dotAll) "$1" bigString
It's quite quick and simple. With this, we can remove unneeded parts over multiple lines. This flag is supported by ES2018 and currently, node.js and browsers other than Internet Explorer and Firefox for Android cover that.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/dotAll

[kl0tl]

Support for this flag is almost on par with the unicode and sticky flags that we already have (see https://kangax.github.io/compat-table/es2016plus/#test-s_(dotAll)_flag_for_regular_expressions and https://kangax.github.io/compat-table/es6/#test-RegExp_y_and_u_flags), so albeit this is an ES2018 feature I think we can merge this.