Falls back to default shell to install plugin on win32

[kwonoj]

related to #1480.

This PR try to fix plugin installation fails on windows machine, if user specifies custom shell other than cmd.exe. The crux of this issue is due to behavior of child_process::exec (https://nodejs.org/dist/latest-v6.x/docs/api/child_process.html#child_process_child_process_exec_command_options_callback), node.js requires any shell specified in windows should compliant to command line parsing with cmd.exe to understand /s /c (or /d /s /c on later version) and does not provide any way to override it.

There isn't seems stable manner of drop-in replacement to child process supports this , so in this PR simply let exec falls back to default shell instead of custom shell when install plugins.

[rauchg]

Are you sure you meant to test for not windows? I don't understand the logic 100%

[kwonoj]

Changed logic is

1. if platform is windows
2. fall back to shell to cmd.exe, by ignoring any custom configuration in hyper.js

if platform is non-windows, it'll still honor config as-is. Main issue in here is node.js force inject /s /c command for windows, assumes all shell specified compliant to cmd.exe, in opposite for non-windows it always inject -c, assumes shell complaint to those switch.

In windows, it is possible to have non-cmd compliant shell. For non windows, afaik it is not possible to have those shell since most known shell even including fish which isn't POSIX-compliant honors -c switch, doesn't need to fall back to default system shell.

[kwonoj]

This is reason in code it test against if platform isn't win32, specify shell as config file.

[dcinzona]

For the sake of conversation, I was having this same issue on my install on Windows 10 with Bash -> zsh set up as my default shell. Including the changes that were part of this PR solved the issue for me.

[rauchg]

Thanks @dcinzona

[jmcbee]

This worked for me. Thank you!