Support for EVAL_RO and EVALSHA_RO

[max-frank]

Noticed that the read only variants for the EVAL commands are not supported at the moment.
The logic for these commands could be mostly the same, but would have to error on calls to "write" functions.

[max-frank]

I might be able to make a PR, but I have not fully understood the LUA implementation so can't make big promises atm.

[max-frank]

Current implementation plan

1. Introduce readonly marker for commands (Add readonly option #415)
2. Update mkLua
   1. accept a readOnly flag indicating readonly mode
   2. add cmd readonly check to top off call/pcall and return readonly error in readonly mode
3. Update eval, evalsha and add RO variants. RO and write variants would call the same function with the only difference being the value of the readonly flag they pass in.

[max-frank]

2 and 3 addressed in #416

[alicebob]

Hi! Thanks for the issue and the PR!, I'll have a look when I have some time.