Handle blank Mix target values properly

[mnishiguchi]

This addresses whitespace or empty string accidentally passed into Mix.target/1.

Changes

• Let Mix.target/1 accept a string value
• Let Mix.target/1 handle whitespace properly
  • trim whitespece
  • do nothing for a blank string

Notes

• Currently users are responsible to validate and create an atom when calling
  Mix.target/1, which is error-prone. For example, :"" can sneak in easily.
• One advantage of Mix.target/1 accepting a string is that we can standardize on converting a string Mix target value to atom properly.
• discussed with @fhunleth at Mix.target() is :"" for ElixirLS nerves-project/nerves#694

[josevalim]

Hi @mnishiguchi! This has been proposed in the past but we don't think it should be the job of Elixir to work around this issue. If something is setting MIX_TARGET to an empty string, then our preference would be to see it addressed. :) In any case, thank you for the PR!