Make pagent.exe path configurable

[Timmmm]

The path to pagent.exe (shouldn't it be pageant.exe btw?) is hard-coded as:

var EXEPATH = path.resolve(__dirname, '..', 'util/pagent.exe');

It would be nice if this were configurable, e.g. for use with Webpack and Electron where __dirname is unreliable and weird.

[rathboma]

Ah, I'm having problems using the pageant ssh agent in my electron app. I wonder if this is why.

Going to experiment and figure out if that's true

[rathboma]

@Timmmm see #894 . I'm fixing this for Electron use cases

[Daemoen]

@Timmmm @rathboma (Not a developer, but guess I will take a stab at it) -- Unfortunately libssh including pageant mandatorily screws up gpg-agent providing pageant. Any suggestions on how to go about handling this use case? (pageant.exe is literally not used for gpg-agent pageant emulation)

[mscdex]

Duplicate of #780.