make readme examples for ES2015 friendly

[zeke]

$(this) will not work within "arrow" functions, where this refers to the lexical this value in the scope of the declaration.

This PR updates removes $(this) from the README in favor of $(el), so the code examples can be rewritten in ES2015 and will still work.

cc @gajus, #762

[coveralls]

Coverage remained the same at 98.832% when pulling 99cedd4 on zeke:replace-this-with-el into 4ccb41b on cheeriojs:master.

[coveralls]

Coverage remained the same at 98.832% when pulling 99cedd4 on zeke:replace-this-with-el into 4ccb41b on cheeriojs:master.

[coveralls]

Coverage remained the same at 98.832% when pulling 99cedd4 on zeke:replace-this-with-el into 4ccb41b on cheeriojs:master.

[coveralls]

Coverage remained the same at 98.832% when pulling 99cedd4 on zeke:replace-this-with-el into 4ccb41b on cheeriojs:master.

[zeke]

whoa, @coveralls 🏇

[tomByrer]

Plus el reads much better than this.

[gajus]

This PR updates removes $(this) from the README in favor of $(el), so the code examples can be rewritten in ES2015 and will still work.

While you are at it, I'd change use of regular functions to arrow expressions.

[zeke]

Bump.

I still think this PR is worth landing, for the sake of making the readme friendly for users of arrow functions.

[jugglinmike]

This change makes the documentation less expressive overall because the this
value of those functions is no longer demonstrated. It's beyond the scope of
this project to help folks learn to use new language features; if consumers use
arrow functions, we have to assume they understand the implications that has on
their code and any examples they find elsewhere. The second parameter is already
listed here (and given the conventional name el), so I think that should be
sufficient.