allow adding of links and own cache

[M1chaelTran]

rather than going down the path of replacing out the links like in #62
we wanted to keep the benefits but enable adding more user define links (eg. apollo-link-state)

for the cache, it can't help as we have to replace.

[zjullion]

Would love to see this merged - as it stands, without the ability to add custom links to the ApolloClient, it's impossible to catch errors from bad credentials (the app just crashes).

It's worth noting that there probably needs to be a slight change to this PR so that if links isn't passed in to the constructor, things still work:
if (!links) { links = [] } or something similar...

[manueliglesias]

Hey @M1chaelTran, @zjullion

This should be possible soon, take a look at #96 and:

• How can I add a custom link to the link chain? #3 (comment)
• Local state story #24 (comment)