Added opengraphs option for open graph tags on docs

[Ephemera]

Usage:

[targets.docs]
title = "Nirum Examples"

  [[targets.docs.opengraphs]]
  tag = "og:url"
  content = "https://nirum.org"

  [[targets.docs.opengraphs]]
  tag = "og:title"
  content = "Nirum Examples"

Closes: #283

[codecov]

Codecov Report

Merging #305 into master will decrease coverage by 0.21%.
The diff coverage is 31.25%.

@@            Coverage Diff             @@
##           master     #305      +/-   ##
==========================================
- Coverage   76.41%   76.19%   -0.22%     
==========================================
  Files          34       34              
  Lines        2883     2899      +16     
  Branches      192      194       +2     
==========================================
+ Hits         2203     2209       +6     
- Misses        488      496       +8     
- Partials      192      194       +2

Impacted Files	Coverage Δ
src/Nirum/Package/Metadata.hs	81.11% <ø> (+0.69%)	⬆️
src/Nirum/Targets/Docs.hs	47.31% <31.25%> (-1.52%)	⬇️

---

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 9fe340d...35dca72. Read the comment docs.

[dahlia]

Build fails:

app/nirum-docs.hs:32:39: warning: [-Wmissing-fields]
    • Fields of ‘Docs’ not initialised: docsOpenGraph
    • In the ‘target’ field of a record
      In the expression:
        Metadata
          {version = Nirum.Version.version,
           description = Just
                           "IDL compiler and RPC/distributed object framework for microservices",
           license = Just "GPL-3",
           keywords = ["IDL", "microservices", "SOA", ....],
           authors = [Author "Nirum team" Nothing Nothing],
           Nirum.Package.Metadata.target = Docs
                                             {docsTitle = "Nirum", docsStyle = style,
                                              docsHeader = "", docsFooter = footer}}
      In an equation for ‘packageMetadata’:
          packageMetadata
            = Metadata
                {version = Nirum.Version.version,
                 description = Just
                                 "IDL compiler and RPC/distributed object framework for microservices",
                 license = Just "GPL-3", keywords = ["IDL", "microservices", ....],
                 authors = [Author "Nirum team" Nothing Nothing],
                 Nirum.Package.Metadata.target = Docs
                                                   {docsTitle = "Nirum", docsStyle = style,
                                                    docsHeader = "", docsFooter = footer}}
   |
32 |     , Nirum.Package.Metadata.target = Docs
   |                                       ^^^^...
nirum-docs.hs: app/nirum-docs.hs:(32,39)-(37,9): Missing field in record construction docsOpenGraph

[dahlia]

Could you append a trailing slash to the url?

[dahlia]

Docs about this new option should be added to docs/target/docs.md file.