Use require.resolve to find plugins in condense.

This makes the bin/condense behavior loading plugins match the bin/tern
behavior, allowing the loading of installed third-party plugins.
Pass distDir to plugin initialize() if present.

This solves the problem of loading Tern modules from plugins outside of the
Tern distribution. Without this change, a plugin does not know where the
Tern distribution loading it is, and attempts to require Tern modules can
either fail or load from the wrong Tern installation (thus failing to
register with the actual running server).

Update the documentation to reflect this change.
Also call plugin.initialize from bin/condense

Issue #365
Issue #351