Support uppercase mappings in visual, but not in visual block mode

Close #23.

As suggested by @Raimondi in https://github.com/wellle/targets.vim/issues/23#issuecomment-35447524 I'm using a visual expression mapping for `A` and `I` that abort early to avoid the delay mentioned in #6.

Only when in character wise visual mode, the mapping reads more characters to look up the appropriate mapping in a local dictionary to support uppercase visual like `v2An,`.