rose task-env --path=[NAME=]GLOB

Support --path=[NAME=]GLOB option in both "rose task-env" and "rose
task-run". New "NAME=" syntax to prepend to named PATH-like environment
variables. NAME is PATH by default.
rose task-env --path=[NAME=]GLOB: syntax fixes.

Syntax fixes after initial testing.
rose task-run: add tests.
rose task-run: now tested with no configuration.
Merge branch 'master' into rose-task-env--path
rose task-run --path=: fix tests.
Fix grammar in documentation.
Merge pull request #666 from matthewrmshin/rose-task-env--path

rose task-env --path=[NAME=]GLOB
Merge branch 'master' of github.com:metomi/rose into metomi-gh-pages