Improve interval strings

[dpsanders]

Towards #206.

Currently implemented:

• @interval("[1]")
• @interval("[-0x1.3p-1, 2/3]")
• convert(DecoratedInterval{Float64}, "[1, 2]_dac")

[dpsanders]

I had to include #211 in this.
Removed again.

[codecov-io]

Codecov Report

Merging #215 into master will increase coverage by 0.04%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
+ Coverage    90.3%   90.35%   +0.04%     
==========================================
  Files          23       24       +1     
  Lines        1001     1026      +25     
==========================================
+ Hits          904      927      +23     
- Misses         97       99       +2

Impacted Files	Coverage Δ
src/ValidatedNumerics.jl	100% <ø> (ø)	✅
src/intervals/conversion.jl	69.56% <100%> (-6.3%)	❌
src/decorations/intervals.jl	84.61% <50%> (-1.88%)	❌
src/intervals/macros.jl	96% <75%> (+0.16%)	✅
src/parsing.jl	96.15% <96.15%> (ø)
src/intervals/intervals.jl	100% <ø> (ø)	✅

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[coveralls]

Coverage increased (+0.04%) to 90.495% when pulling 74daca6 on interval_string into 69bd570 on master.

[coveralls]

Coverage increased (+0.04%) to 90.351% when pulling 8c83fb7 on interval_string into daba4ef on master.

[coveralls]

Coverage increased (+0.04%) to 90.351% when pulling 2b72010 on interval_string into daba4ef on master.

[coveralls]

Coverage increased (+0.04%) to 90.351% when pulling 7cc6e29 on interval_string into daba4ef on master.

[coveralls]

Coverage increased (+0.04%) to 90.351% when pulling 7cc6e29 on interval_string into daba4ef on master.

[lbenet]

LGTM. Nice work! I'll merge it.