Add source-based task runner

[tkf]

Hi, thanks for pulling my last PRs.

I wrote some elisp to support "task" definition in eproject. For example, you can run make command from anywhere in the project directories using this. Each language (project type) can support specific tasks (see eproject-python.el for example). It uses source-based (like helm/anything) so user can extend it.

I defined three sources:

• eproject-tasks-source-metadata

  It reads configuration from eproject type metadata. So it can be used to define "language specific" tasks.

• eproject-tasks-source-path

  You can define tasks per project root (matched using regexp). This can be used to define project-specific tasks.

• eproject-tasks-source-compile

  It detects Makefile at the project root directory and you can simply run make command against it.

[jrockway]

This is really well done. Thanks for contributing!