Add isForced argument, which forces respawn even without node flags

[sttk]

This pr adds isForced argument to this module. isForced is to force respawn even if forcedFlags is empty or is not specified.

I will modify Liftoff to respawn itself when cwd or configPath is specified explicitly, in order to add node_modules in cwd to environment variable NODE_PATH. (process.env.NODE_PATH is not allowed to change in a program).

By this, for example, gulp can load modules in node_modules in both gulpfile directory and cwd. (And this will solve the issues gulpjs/gulp#2126 and gulpjs/liftoff#55.)

[phated]

@sttk I didn't get a chance to respond in full to your suggestion but I don't like the idea of introducing this sort of magic into the CLI. If we do this, we're basically warping the way node's require works. Maybe we just need to better document this edge case and suggest the consumer add NODE_PATH before their command?

[sttk]

@phated You're right. This way is certainly odd, and I agree to placing that suggestion on document.

I'll close this pr.