Extended Eval Plugin: we need more precise handling of `:set`ting flags

[konn]

As discussed in #1232, the current implementation of :set command in Eval Plugin is somewhat ad-hoc, and several extensions involving digits (such as Rank2Types) are rejected and there is no support of other style flags, such as -fprint-explicit-foralls.

We can use parseDynamicFlagsCmdLine to remedy this situation and comply with the spec of flags used in GHC.