add --no-flag for --flag boolean flags

[cppforlife]

may be add a configuration negavtive:"true" to the DSL to indicate which flags should receive --no-* automatically? wdyt?

[jessevdk]

The design of boolean flags has been that you only ever can turn them enabled, not disabled. See #80 and #175 for previous discussion.

[prashantv]

I'm trying to use the ini support as a "default" file, where we enable a boolean flag, and need an easy way to disable it from the CLI.

Using a custom Bool type adds some overhead -- now the user always has to specify --flagname=true rather than just --flagname. The ideal option would be using --no-flagname, but this is not easy to do outside of the library. We'd have to create a second flag, post-process the struct to flip flagname to false, etc.

Having a configuration option in Options to automatically register --no-flagname for booleans would be ideal, does that seem OK to you?