Allow "." to be escaped

[mtdowling]

A lot of JSON keys contain dots. I suggest adding a "" escape character for dots to allow them embedded within keys.

[mtdowling]

Actually, I think "" should be added an escape character for any special character. I think the regex for an identifier should be expanded to match the JSON spec (which means we need to allow the special characters for JamesPath expressions to be escaped).

[jamesls]

I think we should match most, but not all, of the JSON spec. Given that JSON strings must be enclosed with " chars, there are some things that don't work as well (namely whitespace chars). I'd also like to reserve most of the non [a-zA-Z0-9] chars for future use, which will allow jmespath to remain backwards compatible if new constructs are added. I'd propose the following slightly modified grammar for an identifier (using the same ABNF form as the JSON spec):

    identifier        = *char
    char              = unescaped /
                        escape (
                            %x20-2F /    ; Space,!,",#,$,%,&,',(,),*,+,comma,-,.,/
                            %x3A-40 /    ; :,;,<,=,>,?,@
                            %x5B    /    ; Left bracket: [
                            %x5C    /    ; Back slash: \
                            %x5D    /    ; Right bracket: ]
                            %x5E    /    ; Caret: ^
                            %x60    /    ; Backtick: `
                            %x7B-7E /    ; {,|,},~
                            b       /    ; backspace
                            n       /    ; new line
                            f       /    ; form feed
                            r       /    ; carriage return
                            t       )    ; tab
    escape            = %x5C   ; Back slash: \
    unescaped         = %x30-39 / %x41-5A / %x5F / %x61-7A / %x7F-10FFFF