add 0.7-compatible showarg methods

[RalphAS]

This change adds versions of the showarg methods compatible with the usage in 0.7 Base.
The old ShowItLikeYouBuildIt versions are still used with older Julia.
Output format is the same as in the earlier versions of Interpolations.
Corresponding tests are re-enabled.

[codecov-io]

Codecov Report

Merging #220 into master will increase coverage by 3.62%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   82.03%   85.65%   +3.62%     
==========================================
  Files          27       27              
  Lines        1002     1046      +44     
==========================================
+ Hits          822      896      +74     
+ Misses        180      150      -30

Impacted Files	Coverage Δ
src/io.jl	83.14% <66.66%> (+83.14%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9c380b0...57109c9. Read the comment docs.

[timholy]

Just noting this is a really crucial PR: we have decided we are not upgrading ShowItLikeYouBuildIt for 0.7, so we need to ditch the dependency and go 0.7+. There are some conflicts here but I am fixing them now.

[tomasaschan]

we need to ditch the dependency and go 0.7+

Is it possible to specify that it is a dependency for 0.6, but from 0.7 and up? Since there are quite a lot of things you need to update in your code to move between 0.6 and 0.7/1.0, I think it would be nice if we could have as long a time window as possible where we support both.

[timholy]

Incorporated into #226