Add configurable placeholder delimiters

[okriuchykhin]

Lets users choose the delimiter style used to wrap field names in templates, instead of being locked to [NAME].

Why

The [ ] delimiter is hardcoded, which clashes with templates that legitimately contain square brackets and rules out the {{ }} style many users expect from other templating tools. This makes the delimiter a configuration choice while keeping the default unchanged.

What

• New ODFReport.delimiters= setting accepting a preset (:square, :curly) or a custom two-element pair, e.g. ["((", "))"]. Default stays :square, so existing templates are unaffected.
• ODFReport.with_delimiters(value) { ... } block helper to scope a choice to a single render and restore the previous value afterward.
• Validation rejects malformed pairs and any delimiter containing the XML metacharacters < > &, since those are escaped in the document and would silently never match.
• All placeholder types (fields, texts, images) pick this up automatically, since they resolve through Field#to_placeholder.

Notes

• Placeholder names are still upper-cased, so under :curly the token for add_field(:name) is {{NAME}}.
• The setting is process-global; documented as not thread-safe for concurrently rendering reports that need different delimiters.

Tests / docs

• Added test/configurable_delimiters_test.rb covering the default, each preset, a custom pair, block scoping with restore, and the validation cases.
• Added a "Configurable delimiters" section to the README.