Add sysconfdir.
Backwards compatibility patch for cross-compilation changes.

Changes back the types of 'configCompiler' and 'configCompilerEx', but marks
them as deprecated. Adds new functions 'configCompilerEx' and
'configCompilerAuxEx'.

The remaining functions that changed type after the cross-compilation changes
shouldn't matter from the backwards compatibility standpoint:

    * InstallDirs.{absoluteInstallDirs, prefixRelativeInstallDirs,
      initialPathTemplateEnv} - the versions in D.S.LocalBuildInfo retain their
      old types and are usually used instead. In any case, removing the Platform
      parameter from here is problematic because the default install dirs now
      include $arch and $os vars.

    * D.S.Configure.configure - shouldn't be used by the setup scripts.

See #1214 for the original (backwards-incompatible) patches.
Feature the 'cabal' tool more prominently in the docs.

The docs/website sorely need to be reworked, but this will do for now.
Remove italic markers, they are parsed incorrectly by Pandoc.

See jgm/pandoc#976.
TOC update.
Docs for 'cabal repl'.
Add docs for sandboxes.
Merge pull request #1430 from 23Skidoo/docs-update

Describe new features in the documentation.
Merge pull request #1429 from 23Skidoo/issue1214-backwards-compat

Backwards compatibility patch for cross-compilation changes.
Merge pull request #1408 from clinty/sysconfdir

Add sysconfdir.