Allow beast::lexicalCast to parse 'true' & 'false' into a bool

Our config and RPC documentation specifies the use of keywords like `true` and `false` but `beast::lexicalCast` and the `boost` variants only support `0` or `1`. Add support for the following: `1`, `true`, `yes` and `on` for `true` and `0`, `false`, `no` and `off` for `false`. 

Reviews: @vinniefalco, @rec