Fix yalc push always adding in pure mode for repos with workspaces

[Blasz]

Fixes #75.

[Nantris]

It's working in pure mode even for a non-monorepo with no yarn workspaces defined. It tells me to use --no-pure to override, but when I do that it still prints out that it's operating "purely."

@wclr

[Venryx]

@Nantris Unfortunately only the "add" and "link" commands are set up to receive/process the "--pure" flag:

• yalc/src/yalc.ts

  Line 170 in 3b834e4

  pure: argv.pure,

• yalc/src/yalc.ts

  Line 186 in 3b834e4

  pure: argv.pure,

If you're using one of the other commands (eg. "restore"), the command doesn't look for the "pure" flag, so cannot pass it down to the underlying functions that it calls. (which I'm assuming is your scenario; I hit this issue when trying to use "yalc restore" in a monorepo, and wondering why "yalc restore --no-pure" had no difference/effect)