Add include_dirs to servedocs

[fredrikekre]

This patch add a new keyword argument include_dirs to servedocs which make it possible to add additional source directories (in addition to docs/src) that should be considered for the file watching. This does, for example, make it possible to watch the source files of your package, and call Revise.revise() in docs/make.jl in order to automatically update docstrings.

[codecov-commenter]

Codecov Report

Base: 74.17% // Head: 74.32% // Increases project coverage by +0.14% :tada:

Coverage data is based on head (a0dc2ee) compared to base (292901b). Patch coverage: 75.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #154 +/- ## ========================================== + Coverage 74.17% 74.32% +0.14% ========================================== Files 4 4 Lines 364 370 +6 ========================================== + Hits 270 275 +5 - Misses 94 95 +1 ``` | [Impacted Files](https://codecov.io/gh/tlienart/LiveServer.jl/pull/154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Thibaut+Lienart) | Coverage Δ | | |---|---|---| | [src/utils.jl](https://codecov.io/gh/tlienart/LiveServer.jl/pull/154/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Thibaut+Lienart#diff-c3JjL3V0aWxzLmps) | `77.27% <75.00%> (+0.44%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Thibaut+Lienart). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Thibaut+Lienart)

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