Add Base.print(::IO, ::Index)

[jakobnissen]

This allows the user to turn an Index into a string easily, and also makes it easier to test the indices.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% :tada:

Comparison is base (36e42cd) 89.72% compared to head (723dd5b) 89.78%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #114 +/- ## ========================================== + Coverage 89.72% 89.78% +0.05% ========================================== Files 15 15 Lines 691 695 +4 ========================================== + Hits 620 624 +4 Misses 71 71 ``` | [Flag](https://app.codecov.io/gh/BioJulia/FASTX.jl/pull/114/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/BioJulia/FASTX.jl/pull/114/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | `89.78% <100.00%> (+0.05%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/BioJulia/FASTX.jl/pull/114?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | Coverage Δ | | |---|---|---| | [src/fasta/index.jl](https://app.codecov.io/gh/BioJulia/FASTX.jl/pull/114?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL2Zhc3RhL2luZGV4Lmps) | `81.25% <100.00%> (+0.53%)` | :arrow_up: |

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