Enable quitting

[mlhetland]

I'm not sure if this is a missing feature or a bug, but at least when I quit Electron (e.g., with cmd-Q or using the menu in macOS), the JSON parsing consistently gives an error:

Unexpected end of input
 ...when parsing byte with value '0'
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] _error(::String, ::JSON.Parser.StreamingParserState{Sockets.TCPSocket}) at /Users/mlh/.julia/packages/JSON/d89fA/src/Parser.jl:150
 [3] byteat(::JSON.Parser.StreamingParserState{Sockets.TCPSocket}) at /Users/mlh/.julia/packages/JSON/d89fA/src/Parser.jl:59
 [4] current at /Users/mlh/.julia/packages/JSON/d89fA/src/Parser.jl:72 [inlined]
 [5] chomp_space! at /Users/mlh/.julia/packages/JSON/d89fA/src/Parser.jl:117 [inlined]
 [6] parse_value(::JSON.Parser.ParserContext{Dict{String,Any},Int64,true,nothing}, ::JSON.Parser.StreamingParserState{Sockets.TCPSocket}) at /Users/mlh/.julia/packages/JSON/d89fA/src/Parser.jl:160
 [7] parse(::Sockets.TCPSocket; dicttype::Type{T} where T, inttype::Type{Int64}, allownan::Bool, null::Nothing) at /Users/mlh/.julia/packages/JSON/d89fA/src/Parser.jl:494
 [8] parse at /Users/mlh/.julia/packages/JSON/d89fA/src/Parser.jl:492 [inlined]
 [9] macro expansion at /Users/mlh/.julia/packages/Lazy/mAoZN/src/macros.jl:256 [inlined]
 [10] macro expansion at /Users/mlh/.julia/packages/Blink/u1xcH/src/AtomShell/process.jl:111 [inlined]
 [11] (::Blink.AtomShell.var"#9#10"{Electron})() at ./task.jl:358

There may be some other way of quitting/terminating the process/window without getting this issue (what is the suggested way of quitting – just quitting Julia?).