Add subproject compile and test commands

[bbatsov]

Summary

Closes #1653.

• Add projectile-subproject-root helper that walks up from the current directory to find the nearest build file (:project-file marker), stopping at the project root
• Add projectile-compile-subproject — runs the project's compile command in the nearest subproject directory
• Add projectile-test-subproject — runs the project's test command in the nearest subproject directory
• Keybindings: c m c (compile subproject), c m t (test subproject)

This works for any project type that defines :project-file — Maven (pom.xml), Gradle (build.gradle), Leiningen (project.clj), etc. The implementation let-binds projectile-project-compilation-dir so the existing projectile--run-project-cmd infrastructure handles everything else (command caching, buffer naming, prompting, etc.).

Example: Maven multi-module project

myproject/
  pom.xml          <- project root
  module-foo/
    pom.xml        <- subproject root (found when editing Foo.java)
    src/main/java/
      Foo.java
  module-bar/
    pom.xml
    src/main/java/
      Bar.java

With cursor in Foo.java, C-c p c m c runs mvn -B clean install in module-foo/ instead of the project root.

Test plan

• Byte-compiles cleanly
• All 210 existing tests pass
• Manual testing with a multi-module Maven or Gradle project