disable figure processing for no-eval chunks

Error origin: when `eval=false` is set, a figure processing need to be disabled, otherwise a formatter tries to process a non-existed figure and falls with a following error:
```
ERROR: key not found: :figure
 in getindex at dict.jl:617
 in format at /.julia/v0.3/Weave/src/formatters.jl:39
 in weave at /.julia/v0.3/Weave/src/Weave.jl:101
 in process_options at ./client.jl:213
 in _start at ./client.jl:354
 in _start_3B_1720 at /usr/bin/../lib/x86_64-linux-gnu/julia/sys.so
```