proc interpolator  + `ProcessBuilder.lines` workaround. Fixes #819

This PR addresses sbt/sbt#819 for 0.14

## ProcessBuilder.lines workaround

1. `ProcessBuilder.lines` is moves  to `ProcessBuilderLines`. (`ProcessBuilder` converts implicitly to it)
2. `stringToProcess` and `stringSeqToProcess` are deprecated.

Existing code using `Process(...).lines` are source compat. For example, Suerethian versioning works as is:

```scala
val gitHeadCommitSha = settingKey[String]("current git commit SHA")

gitHeadCommitSha in ThisBuild := Process("git rev-parse HEAD").lines.head
```

### 0.14?

I've used "0.14.0" in deprecation annotation. Don't merge this into 0.13 branch.
Here's from the newly added `ChangeSummary_0.14.0.rst`:

`proc` interpolator
-------------------

`proc` interpolator was added to create `ProcessBuilder` in the build definition.

::

    val gitHeadCommitSha = settingKey[String]("current git commit SHA")

    gitHeadCommitSha in ThisBuild := proc"git rev-parse HEAD".lines.head

Each expression embedded in `proc` strings are treated as an argument, so this could be used to pass strings with white spaces. One exception is expressions of type `Seq[Any]`, which is expanded into a list of arguments.

::

    val grepFoo = taskKey[Unit]("grep for def foo")

    grepFoo := {
      proc"grep ${"def foo"} ${(sources in Compile).value}".!
    }

This is useful when implementing input tasks.

::

    val git = inputKey[Unit]("git")

    git := {
      import sbt.complete.Parsers._
      val args: Seq[String] = spaceDelimited("<arg>").parsed
      proc"git ${args}".!
    }